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[ 1-73 ]

VI f. On the Intestinal Tract of Birds ; with Remarks on the Valuation and Nomen-
clature of Zoological Characters. By P. CHALMERS MITCHELL, M.A., D.Sc. Oxon.,
F.L.S., F.Z.S., Lecturer on Biology at the London Hospital Medical College.

(Plates 21-23.)

Read 21st March, 1901.

CONTENTS.

Page

Introduction 173

The Tract in Palamedea cornuta as an Archecentric Type 175

Valuation and Nomenclature of Characters 178

SYSTEMATIC DESCRIPTION 182

Ratitse 182

Carinatas 184

f Colymbiformes 184

J. Sphenisciformes 186

I Procellariiformes 188

f Cicomiformes 189

•< Anserif'ormes 201

[ Falconiformes 205

Summary of the Pelargo-Colymbomorphino Brigade 213

(" Tinamiformes 215

I Galliformes 216

1 Gruiformes 222

[ Charadriiformes 230

Summary of the Alectoromorphine Legion 240

f Cuculiformes 242

•| Coraciiformes , 245

( Passeriformes 252

Summary of the Coraciomorphine Legion 256

Summary of the Systematic Description 257

Autonomic Nervous System 258

Morphology of the Intestinal Tract 261

MeckeFs Diverticulum 262

Meckel's Tract , . . . 264

Supra-duodenal Loop 265

Colic Caeca 267

Characters and Classification 27D

Bibliography 273

Explanation of the Plates 275

INTRODUCTION.

IN 1813, in one of a series of papers on the Solvent Glands and Gizzards of Birds,
Sir E. Home (15) gave a description and some measurements of the intestines in the
Ostrich, Emu, Cassowary, and Rhea; and in 1814 (16) the same author described the
course of the intestines and the form of the cseca in a number of birds, giving figures, in
which, however, the coils were represented as freed from their blood-vessels and mesen-
teries. In the second edition of the ' Le9ons ' Cuvier (5) described the arrangement of the
intestinal tract in some dozen birds and distinguished regions, as the " duodenal loop " ;

SECOND SERIES. — ZOOLOGY, VOL. VIII. 28

174 DE. P. CHALMERS MITCHELL ON THE

the " median loop," extending from the duodenum to the unpaired caecum ; the " colic
loop," extending from the median loop to the insertion of the paired caeca ; and the
" rectum." Following Meckel, he identified the ".unpaired caecum " as the rudiment of
the yolk-sac. Cuvier, however, had no great range of facts before him, and refrained
from any general conclusions. Owen, in Todd's ' Cyclopaedia of Anatomy and Physiology '
(31), added little to the investigations of Home and Cuvier; and Macgillivray (22) did
little more than to point out that the subject might yet afford useful facts for taxonomy.
Thereafter the subject was apparently completely neglected until Dr. Gadow (10, 1 1, 12)
began his extensive and extremely interesting investigations. Gadow studied and
figured the loops and blood-vessels in a large number of forms, distinguished the loops
of the intestine as being " open." or " closed " according to the width of the mesentery
between the limbs of the fold, named the folds " right-handed " or " left-handed "
according to the position of the descending limb ; but, above all, described at length
and based taxonomic distinctions on the mode in which the loops were folded within the
coelom.

In 1894, working in the Prosectorium of the Zoological Society, I examined the
intestines of a number of birds in the method from which Gadow obtained his notable
conclusions ; but I found not infrequently that there were individual variations in the
disposition of the loops, particularly as regarded their " right "- and " left-handedness,"
no doubt in connection with the writhing movements of the gut during life, and
came to the conclusion that there was more to be learned from investigation of the
relation of the loops to the unfolded mesentery than from consideration of the mode in
which the folds were packed. The method of examination, which I have since found
to be an extension of Cuvier's method, I described in 1895 (25), and, at greater length,
in 1896 (26). It depends on the morphological nature of the intestinal tract of Verte-
brates, which is a tube suspended in the ccelom by a fold of mesentery attached to the
dorsal wall. The tube, in its course, describes an arc between two fixed points, the
pyloric end of the ^Stomach anteriorly, and the insertion of the rectum to the cloaca
posteriorly. A third fixed point is given in the embryonic stage by the attachment of
the yolk-sac nearly in the middle of the ventral edge of the arc, and this point is often
marked in the adult by Meekcl's diverticuluru (see infra, p. 175). The intestinal tract
increases considerably in length between the fixed points, and along with its mesentery,
which similarly increases, it is thrown into a series of loops which are folded on one
another in the various modes described by Gadow. When the intestines have been
removed from the bird by section at the pylorus and cloaca and by cutting the mesentery
along its dorsal attachment, the cut end of the duodenum and of the rectum may be
pinned on a board to the operator's right-hand, these two points and the cut edge of the
mesentery stretching between them being placed in their natural relative positions. Then,
with some trouble in the more complicated cases, the various loops may be unfolded
to the left and pinned out ; whereupon the mesentery appears as a flat sheet, in shape
roughly the segment of a circle, the cut dorsal edge of the mesentery being the sector,
and the arc, which carries the intestinal tract, being irregularly distorted. The sheet of
mesentery is of course double, and the blood-vessels and autonomic nervous system run
between the two sheets, being situated outside the coelom. The figures which illustrate

INTESTINAL TRACT OF BIKDS. 175

this memoir represent the intestinal tracts of various birds as seen in preparations of
the kind described, and therefore illustrate the morphological conditions of the tracts.
In 1896 (26) I had already a considerable material, and described and figured some of
the chief modifications of the common type displayed in the groups of birds. Since
then I have been able to add largely to my material. I have now examined many
hundreds of birds, including a number of rare forms, and representing nearly all the
important groups of birds. Taking the System given by Dr. Gadow in Bronn's ' Thier-
Beich,' vol. vi. part ii. pp. 299-301, as one specially familiar to anatomists, I find that my
material includes, so far of course as living forms go, all the groups of B/atites, and, of
Carinates, all the Orders, all the Suborders except Mesites, and all the Families except
MesitidaB, Galbulidse, and some Passerine families. For materials I am indebted to the
authorities of the National Collection, who have allowed me to examine a number of
spirit-specimens, to several private persons, including in particular Mr. C. Hose of
Borneo through Mi\ Beddard, but above all to the Zoological Society of London.
Nearly all the work was done at the Prosector! um on materials from the Gardens or
sent to the Prosector. To my friend the Prosector, Mr. Beddard, F.R.S., I am deeply
indebted both for materials and assistance. To Prof. Howes, F.R.S., I am indebted for
many valuable references to literature, and I have had the- advantage of discussing the
nomenclature of characters with Prof. Ray Lankester, F.K..S.

THE TRACT IN PALAMKDEA COBNUTA AS AN ARCHECENTRIC TYPE.

It is convenient to describe the conditions of the intestinal tract in one form in some
detail so as to provide a standard for comparison and to avoid unnecessary repetition.
In a former paper (26. p. 138, fig. 2) I took the condition in an embryo of an Argus
Pheasant, about thirty days old, as a starting-point ; here, for various reasons which will
appear later, I begin with the condition in the Horned Screamer, Palamedea cornuta
(fig. 1). When unfolded, in the fashion already described, the intestinal tract is seen
to present three main portions between the pyloric cut end (p.) and the cloacal cut
extremity (cl.}. The first division from^. to the point where, at p.v., the cut portal vein
is represented consists of a long, narrow, and closed loop ; this is the duodenal loop of
Cuvier : it contains the pancreas between its ascending and descending limbs, and
receives the ducts of the pancreas and liver. It always in the undisturbed condition
lies bent backwards towards the cloaca, ventrad of all the other loops, and is therefore
the first portion of the gut to be seen when the abdomen is opened from the ventral
surface in the usual fashion. Its ascending and descending limbs are held together by
a narrow outgrowth of the mesentery. The mesentery, after leaving the duodenum,
expands into a large, nearly circular fold, at the circumference of which, from the
end of the duodenum to c.c., the point of insertion of the caeca, is suspended the great
portion of the intestine. About the middle of this portion is situated Meckel's Diverti-
culum (m.\ which is well known to be a relic of the embryonic stalk of the yolk-sac
(cf. 26. fig. 2). This portion I name Meckel's Tract, as it carries Meckel's diverticulum.
In Palamedea it presents a number of minor, simple corrugations, but in more specialized
forms the loops present extremely definite and well-marked types of divergence from

28*

176

DE. P. CHALMEES MITCHELL ON THE

this simple condition. Cuvier (5) divides this median portion into an "Anse moyenne,"
extending from the duodenum to Meckel's diverticulum, and an " Anse colique," from
the diverticulum to the insertion of the caeca. The examination of a larger number of
types, however, shows that although the remains of the yolk-sac when present give a
point of orientation, still there is not a natural point of division between the loops
at the insertion of the yolk rudiment, which, indeed, most frequently lies at the
summit of a minor loop. Moreover, the adjective " colic " rightly belongs to a lower

Fig. 1.

Intestinal Tract of Palamedea cornuta. From p. to p.v. is the Duodenum ; from p.v. to c.c., the insertion of the caeca,
is Meckel's Tract ; from c.c. to cl. is the Large Intestine, here convoluted in its anterior portion, O *.

p.v., portal vein ; a.m.v., anterior mesenteric or duodenal vein ; m.m.v., middle mesenteric rein'; p.m.v., posterior
mesenteric or rectal vein ; ,r, " bridging " factor of duodenal vein from cseca ; R, recurrent factor of middle
mesenteric vein ; p., pylorus ; m., Meckel's diverticulum ; el., cloaca ; n.n., nerves entering mesentery ; v.n.,
" visceral nerve," ganglionated chain of the autonomic nervous system.

portion of the gut, as the colon of mammalian anatomy, from which the name was
taken, is not the portion on which Meckel's diverticulum may be found. It is worth
noting, however, that very often a change in the colour oi' the gut, denoting a phase in
the digestive processes, begins just distal to the diverticulum. Meckel's tract, in tlie

* Since the large intestine is, with few exceptions, short in birds and but rarely differentiated into anything
comparable to a colon, it will be simpler to refer to it in this Memoir as rectum.

INTESTINAL TKACT OF BIEDS. 177

undisturbed condition, lies very irregularly folded, or rather crumpled-up dorsad of
the duodenum and veutrad of the rectum. Its posterior portion is nearly straight,
an extremely common state. The rectum or third portion of the gut extends from c.c.,
the end of Meckel's tract, to cl., where it enters the cloaca. It is suspended by a long
straight piece of mesentery continuous with the mesentery of Meckel's tract. In the
figure, as in some of the later cases, the rectum is represented as folded over to the right
so that the cut edge of the mesentery from the duodenum to the cloaca is twisted on
itself at the point where the caeca are inserted. The large intestine in most birds is
extremely short; its condition in Palamedea, where it is convoluted and might be
divided into colon and rectum, is unusual. The calibre of the gut varies : the condition
seen in Palamedea is very usual ; the duodenum is wider than Meckel's tract, and the
rectum is wider than either that or the duodenum. The caeca are equal in size and of
moderate length, being intermediate between the short thick stumps of Passerines and
the extremely long, irregularly dilated pouches of the fowl. The blood-vessels, shown
in black in this and the subsequent figures, are the veins. At p.v. is represented the
cut edge of the main portal vein on its way to break up in the liver ; the large factor
from the stomach and spleen is not represented, but the three characteristic intestinal
factors are figured. These are the anterior mesenteric or duodenal, draining the
duodenum, and at x receiving a small factor which runs forwards from the caeca and
posterior part of Meckel's tract. The draining of these parts of the intestine by a
tributary or tributaries of the duodenal vessel is seen here in a simple form, but, as will
be seen later, becomes a peculiar and important structural feature. The second or
middle mesenteric factor of the portal drains Meckel's tract; its main axis runs from
Meckel's diverticulum, but a strong factor, which I call the " recurrent mesenteric,"
curves round from backwards following the contour of the tract. The posterior mesenteric,
or rectal factor, drains the rectum ; in many birds it arises from two factors which run
backwards along the ventral surface of the kidneys and join opposite the junction of
the rectum and cloaca, entering the rectal mesentery at that point. These three
components of the portal correspond to the three main divisions of the intestines — the
Duodenum, Meckel's tract, and the Rectum ; they meet nearly at the same point, but,
in different birds, the order of their joining to form the common portal differs ; and,
after giving considerable attention to the matter, as I cannot find significance in the
order of their junction, I shall not refer to it in this memoir. The arteries of the
mesentery are small relatively to the veins; their minor branches follow the minor
branches of the veins fairly closely ; the main branches arise from the dorsal aorta.
The dotted chain marked v.n. in the figure is the part of the " autonomic nervous
system " which follows the intestine. It is extremely plain in some birds, as in
Palamedea, where it consists of a ganglionated chain following the general contour of
the intestinal loops. The nature of my material has not allowed me to follow it out in
the majority of my specimens. To certain general points in relation to it I shall recur
later.

178 DR. P. CHALMERS MITCHELL ON THE

VALUATION AND NOMENCLATURE OP CHARACTERS.

In the description of the structure of an organ or anatomical part as it occurs in a
large series of different forms, it becomes necessary from simple convenience to attempt
some kind of valuation according to which the series of facts shall fall into definitely-
named groups. When there is attempted the difficult passage from descriptive anatomy
to morphology, it is necessary that the valuation and nomenclature should be in relation
to the theory of descent with modification. I assume that birds were monophyletic in
origin, and that the existing forms have branched out in diverging directions from the
ancestral group. The members of this ancestral group, at the stage when they first
might have been called birds, possessed an heritage of characters and tendencies, and these
characters and tendencies have undergone modifications different in amount and nature
in the different groups. The first business is to come to a decision as precise as possible
as to the ground-plan, or archetype, the most ancestral condition of the structures under
consideration. In the present case, I find that the condition of the gut in Palamedea
(fig. 1) may be taken (after allowance for its length, as it is a large bird and vegetarian)
as representing closely the ancestral type. The form of the gut is extremely simple ;
it is distinguished from the intestinal tract of reptiles chiefly by the fact that the three
divisions — the duodenum, Meckel's tract, and the rectum — are sharply marked off one
from the other. Meckel's diverticulum, the morphological median point of Meckel's tract,
occurs nearly at the actual median point. The pair of ca?ca are of moderate length and
are functional. The arrangement of the veins is also extremely simple. It will be seen
in the systematic part of this memoir that it is not difficult to refer the more complicated
types of arrangement of the tract to the condition in Palamedea. I propose to call such
a condition " archecentric," implying that it represents a primitive, ancestral, or central
condition, from which the conditions to be found in the other cases have diverged. It
is obvious that the possession by two or more groups of birds of a character in its
archecentric form cannot be an indication in itself that these groups are more closely
related to one another than they are to groups possessing the character in another form ;
for if the diagnosis of archecentricity be correct, the condition has been present in all
birds, and may be retained by any. For example, I have recently (28, 29) endeavoured
to show that the condition in the wing known as diastataxy is archecentric ; that is to
say, that in the ancestral wing there was a gap in the series of quills proximad of the
fourth secondary quill. If that be correct, the fact that two groups of birds possess
diastataxic wings is no reason for uniting the groups.

When the ancestral condition is modified, it may be regarded as having moved out-
wards along some radius from the archecentric position. Such modified conditions I
propose to call " ap^centric." Again, it must be obvious that the mere apocentrioity of
a character can be no guide to the affinities of its possessor. For instance, in the work
on the wing of birds, to which I have already referred, I tried to show that the condition
of the wing known as eutaxic is apocentric; that is to say, that it is a modification of
the archecentric condition, which in this matter is the condition termed diastataxic.
Before deciding as to the value of eutaxy in a natural classification, it would be necessary

INTESTINAL TEACT OF BIRDS. 179

to decide whether the modification of the archecentric condition were a simple change
that we might expect to occur in independent cases, or if it involved intricate and
precisely combined anatomical changes that we could not expect to occur twice inde-
pendently. In fact, having come to the conclusion that a character is apocentric, we
must pass on to consideration of the problem whether or no the apocentricity be uniradial
or multiradial. In the case of eutaxy I came to the conclusion that it was the result of
a simple closing of the quill series, which might have occurred repeatedly, and probably
did occur repeatedly, and that therefore it was an instance of what I call here " multi-
radial apocentricity." Similar and common multiradial apocentricities, from which no
direct argument of kinship is to be drawn, are to be found in conditions depending on the
degeneration of a structure. If, for example, the presence of basipterygoid processes
be archecentric in birds, their absence is a multiradial apocentricity from which no
direct argument as to affinity may be drawn.

These multiradial apocentricities lie at the root of many of the phenomena that have
been grouped under the designation " Convergence." Especially in the case of manifest
adaptations, organs belonging to creatures very far apart genealogically may be moulded
into conditions which are extremely alike. It is of course the business of science to
distinguish the dissimilarities of genetic material under the similarities which are the
results of adaptation to a common purpose. Ray Lankester (20) long ago designated
such parallelism of modifications as Homoplasy ; and there seems little reasoa to doubt
that a careful discrimination and elimination of the homoplastic features in so-called
cases of convergence would leave a residuum plainly showing the genetic differences,
and dispelling many of the hazy ideas which have been grouped round the word
convergence. In the case of the alimentary canal, it is easy to set apart certain
modifications as directly adaptive, and as therefore of no value when the character of
an organ is being considered as an indication of the natural affinities of its possessor.
Eor the apocentric modifications in question have been produced in different mammals
as well as in different birds, and hence in birds these modifications must be multiradial
and no indication of relationship. I find in the intestinal tract of birds four plain
homoplasies — that is to say, four kinds of adaptation which produce multiradial apo-
centricity ; three of these are well known, the fourth, so far as I am aware, has not yet
been pointed out. That all four occur among mammals as well as among birds is a
fortunate circumstance that definitely proves their multiradial character.

First Homoplastic Modification, Lengthening of the gut in graminiferous and grazing
birds.— This feature has been well known for long, and is in obvious relation to the
circumstance that the nutrient constituents of the food of such creatures are in a form
difficult to digest and in intimate admixture with a large bulk of indigestible material.
Much time and much surface are required for digestion and absorption, and these are
provided for by increase in length of the gut.

Second Homoplastic Modification. Lengthening of the gut with thickening of its
wall and relative decrease of its calibre in piscivorous birds. — The very long and narrow
gut with stout walls is known in birds and mammals. The small calibre and thick wall
are doubtless a protection against mechanical injury by sharp bones. The extretnw

180 DR. P. CHALMERS MITCHELL ON THE

length is at first surprising, as we are accustomed to regard fish as being readily digestihle.
However, a bird does not eat fish carefully with a knife and fork, but bolts it whole.
Dr. Hutchison, a recent writer on foods (17), calls attention to the large amount of
waste matter in uncooked fish, amounting to fully seventy per cent.

Third Homoplastic Modification. Shortening of the gut in frugivorous birds. — The
tendency of the gut in birds and mammals which live chiefly on fruit to be very short,
thin-walled, and wide is well known, and is in direct relation to two simple physiological
factors. The nutritious substances in fruits are in a form which renders them capable
of rapid and fairly complete absorption, and the organic salts present stimulate osmosis.
The ease of absorption makes a relatively large surface unnecessary, and the large calibre
of the gut not only diminishes the outflow from the blood caused by the presence of
organic salts, but it decreases the danger of violent purging. The vigour of peristalsis
in birds is remarkable ; when the duodenum of a pigeon contracts it becomes as hard
and tense as a piece of cartilage.

Fourth Homoplastic Modification. Increase of length rather than of calibre in large
birds. — I am not aware that the circumstance has been noticed, but it is the case that
in large birds and large mammals the gut tends to be relatively longer than in small
birds and small mammals. The explanation, I think, is simple. In the course of
phylogenetic increase in size, the various organs increase in size correlatively with the
whole, but in a mode corresponding to their functions. The first business of the.
intestine is to present to the food-absorbing surface sufficient to supply the needs of the
whole body. As the calibre of a tube increases, its capacity increases more rapidly than
its surface ; it follows that to preserve the same relation of intestinal surface to intestinal
capacity, the length of the gut must increase more than the calibre in the course of
phylogenetic increase of size. It is interesting to notice that the caeca and the rectum,
two portions of the gut of birds in which absorption is not so great, increase in large
birds almost as much in calibre as in length, so that very frequently large birds display
CEeca and rectum which appear to be much wider than the anterior portions of the
intestinal tract. It is plain, from what has been said, that no genetic significance is to
be attached to such conditions, which are merely a result of the homoplastic modifi-
cation due to large size of the whole creature.

There is no need to discuss here the difficult problems as to the mode of origin of
homoplastic resemblances. To some extent they may be freshly epigenetic in each
generation; and Gadow's (12) investigations into the lengths of the gut in chicks as
compared with adults would seem to show that there is much to be learned as to the
occurrence of changes of form and length in direct relation to changes of diet. They
may have come about by a slow selection of strains with genetic variations in the
direction of increased length or of capacity to acquire increased length at the stimulation
of food; or, on neo-Larnarckian principles, they may be the summations of the effect of
stimulations in a series of generations. It is enough to state that these homoplastic
modifications must be allowed for or " corrected " before the drawing of conclusions as
to relationship. Before the condition of an intestinal tract can be taken as affording a
clue to the affinities of its possessor, it must, in imagination, be shortened, in the case

INTESTINAL TRACT OF BIEDS. 181

of a large bird, or a graminiferous or piscivorous bird, or lengthened in the case of a
frugivorous bird. In more general terms : when we are satisfied that an apocentricity
is multiradial, as is certainly the case when it is homoplastic, \ve must neglect it when
we are dealing with the one character as a guide to affinity (fig. 2).

Fig. 2.

V-<

MfTACENTRE .; r — *

" \
MULTIRADML '

APOCENTRIC

PSEUOOCENTRIC

rUNIHADIAl
' APOCENTRIC

I ARCHECENTR/C I

Diagram to explain Nomenclature of Characters.

A complex npocentric modification of a kind that we cannot well expect to be repeated
independently, and that may be designated as uniradial, must be the most certain guide
to affinity. It happens frequently that such a modification forms a new centre around
which new diverging modifications are produced. Such a centre I propose to call a
" Metacentre," borrowing a convenient term from physics. It is obvious that the
condition of a character, archecentric so far as the whole group of birds is concerned, is
metacentric with regard to the common stock of birds and reptiles, and that the trans-
formation of an apocentric character into a metacentre is simply an event in the general
process of divergent evolution. I justify the nomenclature which I am proposing
largely because it brings the valuation and classification of characters into line with our
conception of the general process of evolution.

Finally, there remains to distinguish a form of apocentricity extremely common and
often perplexing. Such conditions are marked by an apparent simplicity that, however,
reveals its secondary nature by some small and apparently meaningless complexity.
Such a condition that mimics the archecentric condition but which can be distinguished
from it, I propose to call " Pseudocentric."

I trust that the ideas underlying this attempt at the valuation and nomenclature of
characters, so far from being novel, are merely a codification of criteria in common

SECOND SERIES. — ZOOLOGY, VOL. VIII. 29

182 DE. P. CHALMEES MITCHELL ON THE

employment among naturalists. I find, however, that such a codification was necessary
when I tried to arrange systematically the modifications of the characters with which
this memoir deals. So far as I have used them in my own work, I have found them
illuminating, and I offer them in the conviction that the rigorous discipline which their
use entails would prove of general utility.

SYSTEMATIC DESCRIPTION.

The Classification according to which this part is arranged is that given by Gadow in
Bronn's ' Thier-Reich ' (12).

RATIT-ffil.

CASUARII. — In the Cassowaries and Emus the gut is archecentric in character. The
three regions are well marked ; the duodenum is wide in the Cassowary, but longer and
narrower in the Emu. In both, Meckel's tract is nearly circular in form, with well-
marked minor folds in the Emu, and, in both, with a distinct relic of Meckel's diverti-
culum about the middle of its course. The rectum is short and nearly straight, this
being the only divergence from the archecentric character and a divergence displayed by
the vast majority of birds. The loops are drained by the characteristic factors of the
common portal vein. The caeca are of moderate length as in the type. I have already
given a figure of the gut in the Cassowary (26. fig. 3).

STRUTHIONES. — The Ostrich (26. fig. 4) is also markedly archecentric. The diver-
gences from the archecentric type are very slight : the duodenum is rather longer and
has a minor loop situated on its distal limb ; the rectum is enormously long, being in
fact at least equal in length to the first two portions together; the caeca also are
extremely long. The factor of the duodenal vein from the caeca comes off rather further
from the main stem of the portal vein, and instead of running forwards within the
mesentery, it leaves the mesentery and in consequence must be separated when the
intestine is unfolded : in this condition it forms what I call a " bridging " vein.

RHE.<E. — In Hhea americana (fig. 3) the gut is archecentric, and indeed can be distin-
guished from that of Palamedea only in two points. Meckel's tract is somewhat
elongated, Meckel's diverticulum lying at its apex, and the caeca are much longer
relatively. The region of the cseca is drained by two " bridging " factors of the duodenal
vein which leave the mesentery and have to be divided when the duodenum is unfolded
outwards. A slight peculiarity that I have noticed in no other bird is that a factor of
the splenic vein (fig. 3, z) drains the proximal portion of the rectum.

APTERTGES. — The gut of the Apteryx (fig. 4), while markedly archecentric, is the
most modified of those among Struthious birds. The duodenum and the rectum conform
to type, save that the rectum is relatively shorter and is straight. The greater portion of

INTESTINAL TKACT OF BIEDS.
Fig. 3.

183

Intestinal Tract of Rhea americina.

x, x', cut ends of " bridging " factors of duodenal vein, draining the distal portion of MeckePs tract and one
caecum ; z, factor of the spleuic vein. General description as in legend of figure 1.

Fig. 4.

x>

Intestinal Tract of Apteryx Mantelti.
For explanation, see legends of figs. 1 and 3. S.D.F., apex of supra-duodenal loop.

29*

DR. P. CHALMERS MITCHELL ON THE

Meckel's tract forms a large circular fold with Meckel's diverticulum at its central point,
opposite the origin of the middle mesenteric vein, as in the archecentric type. But the
posterior portion of the tract, that to which the very long caeca are attached, is specialized
into a distinct minor loop. This I call the supra-duodenal loop, because, in the natural
condition of the intestine, it lies directly above the duodenum and is drained by factors
of the duodenal vein. This loop in Apteryx is the first appearance of a specialized
structure to which I shall have to direct repeated attention.

When correction has been made for the obviously homoplastic modifications in the
intestinal tract of Ratites — that is to say, when a reduction has been made in the case of
the larger forms for size and in the case of the herbivorous forms, — it is plain that while
all are archecentric, the Casuarii are by far the least modified, and that in this as in
many other characters they deserve the position assigned them as extremely primitive
types. Struthio and Rhea are more modified, and the modification is in the same
direction in each. Apteryx stands somewhat apart from the others, the specialization of
the supra-duodenal loop being well marked in it- The general character in all, however,
is so plainly archecentric that no argument can be drawn from the intestinal tract for or
against the polyphyletic nature of the group. Such similarities as they present to one
another or to other groups are simply part of the common inheritance of all birds.

COLYMBIFORMES.

COLYMBI. — I have examined the Great Northern and Red-throated Divers (Colymbus
glacialis and C. septentrionalis), and have already figured the intestinal tract (26. fig. 5).
The duodenal Icnp is straight and normal; the rectum is straight and very short.
Meckel's tract is pulled out into a series of short, straight, symmetrically arranged loops
with the Meckel's diverticulum nearly at the middle of its course, and with a well
specialized supra-duodenal loop drained by a bridging factor of the duodenal vein. The
three main factors of the portal vein are typical, and the caeca are of moderate length.
It is clear that the Divers present a certain degree of apocentricity in the gut. The
fish-eating habits have lengthened the tract, but the increase of length takes place not
by a series of irregular loopings but in a definite fashion.

PODICIPEDES. — In the Crested Grebe, Podicipes cristatus (fig. 5) the duodenum is a
large loop somewhat wider distally. Meckel's tract is still more specialized than that of
the Colymbi. It presents first a long minor loop, then a short loop, then a long loop,
on which, from the arrangement of the blood-vessels, I suppose the yolk-sac to have lain,
although it is not represented by a Meckel's diverticulum ; then follow two short loops,
and then a very large supra-duodenal loop with characteristic bridging vein. The caeca
are relatively short, but wide and functional, and the rectum is short and wide.

In Tachybaptes fluviatilis, the Little Grebe (fig. 6), the apocentricity seen in the
Crested Grebe is carried further in two points, otherwise the character of the tract is

INTESTJNAL TEACT OF BIRDS.

185

similar. Two of the minor loops of Meckel's tract are carried out to a great length, one
of them bearing Meckel's diverticulum at its summit ; the caeca "are relatively much
longer, and the supra-duodenal loop is still more highly specialized. The Podicipedes,
then, present an apocentricity rather more marked than that of the Divers.

Fig:. 5.

Intestinal Tract of Podieipes cristatus.

x, bridging factor of the duodenal vein draining S.D.F., the supra-duodenal loop ; ?, probable

position of Meckel's diyerticulum.

When allowance has been made for the size and piscivorous habits of the Colymbiform
birds, it appears that while the character of the gut has moved outwards from the arche-
centric position chiefly in the direction of the formation of specialized straight loops
(according to Gadow's nomenclature, the folding is orthocoelous), the apocentricity is not
great, and is certainly not of such a uniradial character as to associate them closely
with any other group.

186

DE. P. CHALMERS MITCHELL ON THE

SPHENISCIFORMES.

I have already described and figured the intestinal tract of Eudyptes (26.^%. 6). I
now am able to add descriptions of the condition in Spheniscus demersus (fig. 7) and
Aptenodytes Pennanti (fig. 8). In all three the intestine is extremely long and of

Fig. 6.

Intestinal Tract of Tachyba^ttes jtuviatilis. Lettering as before.

slender calibre with firm walls; in the figures the actual length is somewhat under-
estimated, so that the " correction " for piscivorous diet has been made partially. The
three portions of the tract are well marked and are each drained by a characteristic
factor of the portal vein. The duodenum is extremely long ; it is thrown into a
complicated set of minor loops resembling the condition in the fish-eating Eagles, in
Eudyptes and Spheniscus ; in Aptenodytes it forms a loosely rolled spiral, a mode of
packing increased length that is not at all uncommon among birds and that must
be regarded as a multiradial apocentricity. Meckel's tract in all three is moderately
specialized ; Meckel's diverticulum lies nearly at the middle of its course, but there is a
strong tendency for the numerous minor loops to be extended in length or expanded

"f.3

INTESTINAL TRACT OF BIEDS.
Fig. 7.

187

Intestinal Tract of Spheniseus demersus.
k, supra-csecal kink ; other lettering as before.

Fig. 8.

m

Intestinal Tract of Apienodytes Pennanti. Lettering as before.

188 DE. P. CHALMEKS MITCHELL ON THE

into minor systems. Of these, two are well-marked in all : a very large loop towards
the distal extremity of Meckel's tract and drained by a factor of the middle mesenteric
vein, and a long narrow supra-duodenal loop which is drained by a " bridging " factor of
the duodenal vein. In Spheniscus the portion between the supra-duodenal loop and the
rectum, which is very long in all, is expanded into a minor fold, an arrangement
characteristic of the Eagles, and to which I give the name " supra-cseeal kink." The
rectum in all is extremely short, and the caeca are vestigial in those that 1 have
examined, but Gadow mentions that they were relatively long in a Eudyptes.

It is plain that the character of the tract in the Sphenisciformes is markedly
apocentric. When due allowance has been made for the piscivorous lengthening, the
form still remains apocentric : the short caeca, the tendency to form minor loops, the
supra-duodenal loop, the tendency to form a supra-caecal kink, and the extremely short
rectum make up a peculiar and distinct type.

PROCELLARIIEORMES.

Of these I have had an opportunity of examining only the Northern Petrel and the
Albatross. In Fulmarus glacialis (26. fig. 7) the duodenum is compound, and Meckel's
tract is expanded into six very long, narrow and straight minor loops, the fourth of
which bears Meckel's diverticulum, while the last is a distinct supra-duodenal loop
drained by a " bridging " vein. The terminal portion of Meckel's tract is long, thrown
into supra-ceecal kinks as in Sphenisctis and the Eagles, and is drained by the posterior
mesenteric vein. The caeca are vestigial, and the rectum is extremely short. In
the Albatross, Diomedea exulam (fig. 9), the duodenum is simple, and Meckel's tract is
drawn out into a series of extremely long narrow loops (the length of these is under-
estimated in the figure). The form of the tract is much alike in the Albatross and the
Petrel ; the chief difference is that the first three minor loops of the Petrel are repre-
sented in the Albatross by two long loops with shorter loops between them. A very
long loop bearing the Meckel's diverticulum then follows in both ; the lower portion
of Meckel's tract, the supra-caecal kinks drained by the rectal vein, the vestigial caeca,
and the very short rectum are alike in each. The only striking difference is that
there does not appear to be a definite supra-duodenal loop with " bridging " vein in
Diomedea. The specimen I examined had been preserved for long in spirit, and
possibly I may have overlooked a "bridging" vein, although I examined the region
minutely for it.

It is plain that the Procellariiform birds present a markedly apocentric type of intes-
tinal tract, the special characters being the transformation of the middle loop into a
number of definitely-placed minor loops, the appearance of supra-csecal kinks on the
large posterior portion of Meckel's tract, drained by the rectal vein, the vestigial caeca,
and the very short rectum.

Gadow (12) unites the Colymbiformes, Sphenisciformes, Procellariiformes, and extinct
Ichthyornithes into a " Legion " under the name Colymbomorpb.se. Concerning the
intestinal tract of the extinct forms we have no information. The others have all moved
out from the archecentric condition. Their apocentricity is first a relatively increased

INTESTINAL TRACT OP BIEDS. 189

length in association with the nature of their food, and certainly multiradial ; second,
Meckel's tract in all, while remaining nearly symmetrical, tends to be drawn out into a
series of long, narrow, and straight loops, a feature which may give some clue to affinity ;

Fig. 9.

Intestinal Tract of Diomeclea exulans. Lettering as before.

third, the rectum in all is very short. The Colymbiformes are least modified; the
Penguins and Petrels are more modified, the two latter showing degeneration of the
caeca and certain peculiarities in the posterior portion of Meckel's tract, peculiarities
repeated in Steganopodes and Falconiformes.

CICONIIFORMES.

STEGANOPODES.

(1) PHAETHONTID.E. — Phaethon (fig. 10) displays a simple form of alimentary tract.
The duodenal loop is simple ; Meckel's tract is thrown into a series of narrow, minor
loops, some of which are slightly complicated by folding. Meckel's diverticulum occurs
on one of the minor loops rather more near the distal end of the tract. The posterior
portion has no special supra-duodenal loop, and ends in a portion drained by the rectal or

SECOND SERIES. — ZOOLOGY, VOL. VIII. 30

190

DE. P. CHALMEES MITCHELL ON THE

posterior mesenteric vein and exhibiting a short supra-csecal kink. The caeca are
reduced, but not to the same extent as in other Steganopodes except the Pelicans. The
rectum is extremely short. The three factors of the portal vein are of diagrammatic
simplicity.

Fig. 10.

Intestinal Tract of Phaethon flavirostris. Lettering as before.

(2) STJLDXE. — In Sula bassana (fig. 11) the apocentricity of Phaethon is carried
further. The general arrangement is similar, but the duodenum is longer and is com-
pound ; certain of the minor loops of Meckel's tract are longer, especially the last, that
forming a characteristic supra-duodenal loop with bridging vein ; and the kink on the
posterior part of Meckel's tract, that drained by the rectal vein, is larger.

(3) PHALACROCORACIDJE. — In Phalacrocorax carlo (fig. 12) the duodenum is very long
and narrow ; Meckel's tract is extremely elongated, in which respect it resembles the
condition found in Platalea and Phcenicopterus, and is symmetrically disposed about the
middle mesenteric vein, which in the archecentric fashion runs from a large Meckel's
diverticulum. The supra-duodenal loop is long; there is a large supra-caecal kink
supplied by the rectal vein. The ceeca are vestigial, and the rectum is relatively longer
that in Phaethon and Sula. In Plotus-anhinga (fig. 13) the form of the tract presents
marked differences. The duodenum is similar. The anterior portion of Meckel's tract
is relatively much shorter, and consists of a single narrow loop, followed by one very

INTESTINAL TRACT OF BIRDS.

191

Fig. 11.

ss>.f

Intestinal Tract of Sula bassa'na. Lettering as before.

Fig. 12.

Intestinal Tract of Phalacrocorax earbo. Lettering as before.

30*

192

DR. P. CHALMERS MITCHELL ON THE

long loop, at the end of which lay what I took to have heen a Meckel's diverticulum.
Then follows a very long posterior portion, relatively longer than in the diagram, and
thrown into a series of straight minor folds, the posterior of which is a supra-duodenal
loop with a " bridging " vein. Just beyond this is the trace of a supra-csecal kink. The
rectum is long and wide, in fact is typically archecentric. Of the usual pair of caeca,
only one was developed, and that in a vestigial form, but Beddard (2) states that indi-
viduals vary, some possessing two vestiges. The minor loops of Meckel's tract show a

Fig. 13.

m .'

Intestinal Tract of Plotus anhinga.
C.C., single colic caecum ; k ?, possible supra-ciecal kink ; m ?, possible position of Meckel's diverticulum.

tendency to be bunched up as in the Pelicans. If I were wrong in my placing of
Meckel's diverticulum, and what I took for it was the merest trace, the character of the
gut would not be so unlike that found in the Cormorant ; but in any event the differences
between the two forms are wide, and make it plain that if other characters justify the
inclusion of Plotus among the Phalacrocoracidae, the association is by no means close.
On the evidence of the intestinal tract I should be inclined to place Plotus in a separate
family.

(4) FREGATIDJ-E. — In Fregata (fig. 14) the form of the intestinal tract is very like
that displayed by Phaethon. The chief differences are that in Fregata the whole tract is

INTESTINAL TEACT OF BIEDS.

193

relatively somewhat shorter, the minor expansions of Meckel's tract are less numerous,
that bearing Meckel's diverticulum being longer ; there is a supra-duodenal loop ; the
caeca are less developed and the rectum is longer.

Fig. 14.

Intestinal Tract of a species of Fregata. Lettering as before.

(5)|PELECANnx£. — In the Pelicans (Pelecanus rufescens, fig. 15) the form of the
intestinal tract is simple. The duodenal loop is long, narrow, and somewhat twisted ;
Meckel's tract is nearly symmetrical round the middle mesenteric vein which runs from
the position of a large Meckel's diverticulum. There is a well-formed supra-duodenal
loop, a supra-caecal kink drained by the rectal vein, and the caeca are relatively longer
than in other Steganopodes. The minor loops of Meckel's tract tend to be bunched up
towards the mesenteric vein, a feature that cannot well be represented in a diagram
showing the unfolded condition.

The Steganopodes are typically piscivorous, although some of them also take any kind
of floating carrion, and in all of them allowance must be made for the piscivorous length
of gut. Their apocentricity, apart from such homoplasy, consists, as in the Colymbo-
morphae, of a general tendency for Meckel's tract to be expanded into a series of short
straight loops. In the Colymbomorphae, however, these loops frequently increase in
length and become reduced in number ; in the Steganopodes the tendency is rather for
the minor loops to increase in number, and for Meckel's tract to be either bunched up

OF THE

UNIVERSITY

OF

194

DR. P. CHALMEKS MITCHELL ON THE

tightly or much elongated as a whole. These two forms of modification of the middle
loop point towards the condition found in many of the Ibididse such as Platalea, and in

Fig. 15.

Intestinal Tract of Pelecanus rufescens. Lettering as before.

the Flamingoes, where Meckel's tract, in the unfolded condition not unlike that of the
Pelican,' is in life twisted irregularly into a spiral. I find, then, in the apocentricity of
the Steganopod Meckel's tract an underlying resemblance to that of the Colymbomorphae,
as if a metacentric position had been common to all these, but from this metacentre the
two sets have diverged in different directions. The reduction of the caeca, the shortening
of the rectum, and the formation of minor loops above the cseca, but drained by the
posterior mesenteric vein, are common, but not invariable, in the whole set.

INTESTINAL TRACT OF BIRDS.

195

ARDE.E.

(1) SCOPID^E. — In Scopus umbretta (fig. 16) the duodenum is long and slightly twisted.
Meckel's tract is nearly symmetrical, a large Meckel's diverticulum being near the centre
of its periphery, and is expanded into a number of wide, irregular, but in the main straight
minor loops, the last of these being longer and forming a typical supra-duodenal loop.
The rectum is straight, of moderate length, and the pair of cseca are reduced. The veins

are typical,
kink.

A small area supplied by the rectal vein corresponds to the supra-caecal

Fig. 16.

Intestinal Tract of Scopus umbrettd. Lettering as before.

(2) ARDEID^;. — In the Herons and Bitterns, of which I have examined a number of
species, the ground-form is like that in Scopus. The duodenum is a long narrow loop,
nearly always considerably twisted to the left, usually more so than appears in the two
figures (Nycticorax griseus, fig. 17, and Ardea candidissima, fig. 18). Meckel's tract
may be rather elongated as in Nycticorax, or relatively shorter as in Ardea. It is
always drawn out into a large number of minor loops, many of which are irregularly
folded, and not infrequently complex in themselves ; the distal minor loops in Ardea
(fig. 18) show this in a relatively simple form. In the Little Bittern and some other

Fig. IV.

Intestinal Tract of Nycticorax griseus.
C.C., single colic caecum characteristic of Herons ; Ic. 1 and Te. 2, supra-cascal kinks. Other lettering as before.

Fig. 18.

m

Intestinal Tract of Ardea candidissima. Lettering as before.

THE INTESTINAL TSACT OF BIRDS. 197

Herons the complexity of the minor loops is very great. The penultimate loop of
Meckel's tract is always a typical supra-duodenal loop with "bridging" vein, and then
follows a supra-csecal kink. It is typical of the Ardeidse, as is well known, that one of
the two colic caBca be absent, and I have found no exception to this, but Beddard has
recorded such a case. The rectum itself is straight, but not very short.

A considerable part of the apocentricity of the intestinal tract in the Ardese must be
attributed to the fact that they are relatively large birds with a diet consisting chiefly
of fish, in consequence of which the gut is very long and narrow. Underlying this is a
general resemblance to the ground-form of the Steganopod and Colymbomorph gut,
shown in the symmetry of Meckel's tract around the middle mesenteric vein, the
persistence of a large Meckel's diverticulum opposite the end of this vein, and the
formation of a kink immediately above the colic caeca but supplied by the rectal vein.
The special Ardeine features are the elongation of the duodenum as a narrow fold curving
to the left ; the throwing out of Meckel's tract into minor loops, which, straight in the
simpler forms and always folded over simply (" orthocoely " of Gadow), tend to become
complicated in themselves. The persistence of only one of the colic ca3ca is a most
peculiar feature, normally absent only in Scopus. But the absence occurs not infre-
quently as an individual abnormality in some other birds, and I shall make further
reference to such cases.

Although Scopus, so far as the character of the intestinal tract takes us, is more
archecentric than the Herons and Bitterns, it plainly belongs to their group rather than
to the Storks and Ibises.

C ICONIC.

(1) CICONIID.E. — The Storks, of which I have examined a considerable number, present
a most interesting series of modifications. Anastomus oscitans (see Plate 21), the Indian
Open-bill, displays so little difference from the ground- form of the intestinal tract among
the set of birds I have been discussing, that I do not think it necessary to figure it in a
separate block. The duodenum is short and straight ; Meckel's tract is thrown into a
very large number of short straight loops nearly symmetrically disposed around
the middle mesenteric vein, the latter running backwards from a large Meckel's
diverticulum. There is a very large and complicated supra-duodenal loop, more compli-
cated than in any of the other Storks I have examined, but drained by the usual bridging
vein. Above the reduced cseca lies a short kink drained by the rectal vein, and the
rectum is straight and of moderate length. In Pseudotantalus ibis (fig. 19) the typical
Ciconiine apocentricity begins to appear, and this is of the definite type that I call
uniradial and am. inclined to regard as a sure sign of affinity. The duodenum is
enormously long, but the bending which appeared in Herons is here transformed to a
spiral twist, represented in the figure as partially uncoiled, with the result that the vein
is out of the mesentery. The first minor loop of Meckel's tract is very large and is in
itself slightly twisted, and, in the unfolded state, partly rolled in the duodenal spiral.
The remaining portion of Meckel's tract consists of a few simple folds, symmetrical

SECOND SERIES.— ZOOLOGY, VOL. VIII. 31

198

DE. P. CHALMEES MITCHELL ON THE

about the middle mesenteric vein, which runs from a Meckel's diverticulum. The distal
portion of Meckel's tract forms first a distinct but short supra- duodenal loop, and
then a double kink drained by the rectal vein. The caeca are reduced and the rectum
is straight and short. Dissura episcopus, Leptoptilus crumeniferus, L. argala, and Ciconia
alba are practically identical with this condition. In Ciconia nigra, which I have already
figured (26. fig. 9), the condition is similar, except that the duodenal loop and the first

Fig. 19.

m...

Intestinal Tract of Psewdotantalus ibis. Lettering as before.

loop of Meckel's tract form more perfect spirals, which in the unfolded condition are
rolled together. In Mycteria americana (fig. 20) a similar condition exists, but the
duodenal spiral and the spiral on the first loop of Meckel's tract are still longer and
more complex, and are more intimately rolled together, with the result that certain
factors of the duodenal vein run across draining the loop with which the duodenum is
associated. These are represented in the figure as divided and the spirals are partly
untwisted. Among the Storks, then, we see the interesting state of affairs that the
character of the gut, starting from a metacentric position common to a large group of
birds, develops along a special radius forming a uniradial line of apocentricity.

INTESTINAL TEACT OF BIEDS.

Fig. 20.

199

Intestinal Tract of Mycteria americana.
z, cut ends of veins from first loop of Meckel'a Tract to duodenal vein.

(2) IBIDID.E. — I have already figured the intestinal tract of Platalea leucorodia
(26. fig. 8). It is more archecentric than that found in other Ciconiiform birds. The
duodenum is a long narrow loop bent round to the left as in the Herons, but showing
no trace of spiral formation. Meckel's tract consists of an elongated system of short
loops, symmetrically disposed round the middle mesenteric vein, which runs from a
Meckel's diverticulum. This region of the gut is slightly twisted into a spiral in the
unfolded condition. The last loop of Meckel's tract forms a simple supra-duodenal fold ;
the caeca are short, and the rectum is straight but considerably reduced. Platalea is
certainly low down in the Ciconiiform scale, but none the less it displays the features of
the group in a simple form, and is quite different in the character of the gut from
Numenius (Plate 22), a low Charadriiform bird with which it has been compared,

31*

200

DK. P. CHALMEKS MITCHELL ON THE

PHCENICOPTERI.

PHCENICOPTEKID^E. — In Phcenicopterus ruber (fig. 21) the duodenum is a simple loop,
but with a slight Heron-like twist. Meckel's tract is very much elongated as in
Platalea, and, again as in that bird, it consists of very many short loops symmetrical
about the middle mesenteric vein which runs towards a Meckel's diverticulum. As in
Platalea this region is twisted into a rude spiral, and there is a well-formed supra-
duodenal loop. The caeca are long, and the rectum is of moderate length but straight.
The only Anserine feature in the intestinal tract of the Flamingo is the presence of long
caeca; and this, as we shall see in the further course of this memoir, is an obviously
multiradial apocentricity. In all other respects the intestinal tract of the Flamingo is
that of a low Ciconiiform bird.

Fig. 21.

Intestinal Tract of Pheenicopterus ruber. Lettering as before.

It is unnecessary to do more than sum up in a few words the conditions of the gut
found among the Ciconiiform birds. The ground-form is closely similar to that of the
Colymbomorphae, pointing towards the existence of a common metacentre for all these
birds. From this metacentre the groups of Ciconiiform.es have diverged in different
directions and to different amounts. The Steganopods show a tendency to the bunching
up of the loops of Meckel's tract on the axis given by the middle mesenteric vein. The
Herons display a tendency to the individual elaboration of the minor loops of Meckel's
tract. The Ciconiine birds show the most definitely uniradial apocentricity, 'consisting
of an elaboration and intimate connection between the duodenum and the first loop of
Meckel's tract, with a reduction of the posterior portion of Meckel's tract, except the
supra-duodenal loop. The Ibididae, or at least Platalea, show a primitive elaboration of
Meckel's tract as a whole, and this tendency is carried further in the Flamingoes.

INTESTINAL TKACT OF BIEDS. 201

ANSERIFORMES.
PALAMEDE^E.

PALAMEDEID^E. — I described the intestinal tract of Palamedea above (fig. 1). The
intestinal tracts of Chauna chavaria (26. fig. 10) and of C. derbiana do not differ from
that of Palamedea in any essential detail. I am following Dr. Gadow's classification for
convenience of reference, but it is obvious that from the character of the intestines the
PalamedeaB have no claim to be placed in close association with the other Anseriform
birds, or indeed with any of the Carinate birds with which I have as yet been dealing.
The Palamedese exhibit what I take to be the most primitive or archecentric type of
intestinal tract to be found among birds. In the gut they have a general resemblance to
the Ratites, and to the lowest members of all the other groups ; to such they bear more
resemblance than to the Ducks and Geese, which in the intestinal tract exhibit a definite
or uniradial apocentricity, no trace of which occurs in any of the three Screamers. On.
the character of the intestinal tract alone I would not remove the Screamers from other
birds, but rather all other birds from the Screamers, leaving them in central and
primitive isolation.

AN SERES.

ANSERID^E. — I have shown that in the Carinates 1 have up to this point discussed,
with the exception of the Palamedeae, there is strong evidence for the existence of the
apocentric type of gut which forms a metacentre from which the various groups have
diverged further. The metacentric character consists chiefly in the transformation of
Meckel's tract into a definite series of narrow, straight loops, folded in the mode Gadow
terms orthocoelous, and arranged nearly symmetrically round the middle mesenteric vein,
which forms an axial line running from a Meckel's diverticulum. The form of the gut
in the Anserida3 can be referred easily to such a metacentric condition. In Anseranas
melanoleuca (fig. 22) this metacentric condition is reproduced with almost diagrammatic
fidelity, an interesting circumstance in view of the primitive position among the Anseres
generally assigned to this bird. The duodenum is a long narrow loop ; Meckel's tract is
thrown into very symmetrically-arranged narrow minor loops, the most peripheral of
which bears a large Meckel's diverticulum. Between this and the duodenum there are
three minor loops, a number typical of this part of the gufc in the Anseres. The last loop
of Meckel's tract is a typical supra-duodenal loop with the usual " bridging " vein from
the duodenum, and with the very long caeca closely applied to it. The rectum is not
very short but is straight. In Cygnus atratus, the tract of which I have figured in a
former paper (26. fig. 11), certain modifications of the type in Anseranas are present.
The first three minor loops of Meckel's tract are more complicated, showing a tendency
to give off secondary loops. The axial loop, which bears Meckel's diverticulum, is greatly
elongated and usually somewhat irregularly folded, with the result that the main vein
leaves the mesentery, being shorter than the loop itself. The minor loop, just posterior

202

DR. P. CHALMERS MITCHELL ON THE

Fig. 22.

1 ntestinal Tract of Anseranas melanoleuca. Lettering as before.

Fig. 23.

m--

Intestinal Tract of Mergus dllellus. Lettering as before.

INTESTINAL TKACT OF BIRDS.

203

to this, is usually elongated and may be similarly twisted. The supra-duodenal loop,
the caeca, and the rectum are as in Anseranas. The vast majority of Ducks and Geese
that I have examined, however different their size and habits, faithfully reproduce this
type. The diagram given for Cygnus atratm, with the most trifling alterations, might
serve for Anas, Anser, ^Ex, Chaulelasmus, Bernicla, Dendrocygna, Fuligula, Nesonetta,
Tadorna, and doubtless, so constant is the type, for many others. Mergus albellus, the
Smew, presents an interesting variation (fig. 23). The duodenum is unusually wide ;
Meckel's tract is thrown into a large number of very short loops at the periphery of an
almost circular mesenteric fold, but Meckel's diverticulum lies at the apex of a somewhat
longer fold lying in the axis of the system. There is a supra-duodenal loop, but it is not

Fig. 24.

Intestinal Tract of Spatula dypeata. Lettering as before.

drained by a " bridging " vein. The colic caeca are paired, but practically non-existent,
and Beddard (2. p. 459) mentions an instance where one of the two was absolutely non-
existent. At first sight this intestinal tract appears more archecentric than that of any
of the other Anseriform birds except the Palamedeae, but I do not doubt that it is an
instance of what I term pseudocentric simplicity. The caeca are obviously degenerate,
and in other Mergansers they are longer ; and examination of the first part of Meckel's
tract shows that it might have been derived by a fusion of the three first minor loops,
these having been more complicated than in Cygnus, and more as they are in Spatula
(fig. 24). There is a short supra-caecal kink, but it is not supplied by the rectal vein.
Spatula dypeata (fig. 24) shows a form of gut which is simply a further elaboration of

204

DE. P. CHALMEE8 MITCHELL ON THE

the Cygnus type. The modifications are confined to Meckel's tract. The three most
anterior minor loops of that region are complicated by an elaborate series of minor
folds; the axial loop bearing Meckel's diverticulum is as in Cygnus, but the loop next
posterior to that is enormously long and folded on itself, the vein having left the
mesentery and running a much shorter course than the loop itself. In Nettopus
coromandeliumis (fig. 25) the gut is relatively shorter, and an apocentric simplification

Fig. 25.

Intestinal Tract of Nettopus coromandelianus. Lettering as before.

has taken place. The three proximal loops of Meckel's tract are represented by one
short and then a very long loop, and the loop next distad of the axial loop is contorted.
In these modified loops the veins leave the mesentery, a condition comparatively rare
among birds, but very common in the Anseres.

Considering them from the point of view of the intestinal tract, it is plain that the
other Anseriformes must be removed from the Palamedeae. The latter are archecentric.
The former start from a metacentric position common to the Colymbomorphse and the
Ciconiiformes, but have diverged apocentrically from that position, forming a specially
Anserine metacentre (Plate 22).

INTESTINAL TRACT OF BIEDS.

205

FALCON1FORMES.

C ATH ART^E.

CATHARTID^E. — Of these I have been able to examine only Cathartes aura (fig. 26).
The duodenum is long and is coiled into an irregular spiral. Meckel's tract presents first
one very wide loop and then a nearly circular expansion symmetrical about a Meckel's
diverticuluni and thrown into short irregular folds. Then follows a very large but

Fig. 26.

Intestinal Tract of Cathartes aura.
Lettering as before.

typical supra-duodenal loop with "bridging" vein. The caeca are totally absent, but
immediately above their normal position lies the kink supplied by the rectal vein. I
have already shown that this peculiar little loop is a recurring feature among the

It occurs without exception in all the Palconiforrn

Carinates we have been considering,
birds.

SECOND SERIES. — ZOOLOGY, VOL. VIII.

200

DR. P. CHALMERS MITCHELL OX THE

ACCIPITKES.

GYPOGERANID^E. — la Serpentarius reptilivorus (fig. 27) the duodenum is simple.
Meckel's tract presents first three narrow minor folds, and then expands into a nearly
circular portion like that in Cathartes, the axis being given by the middle mesenteric
vein which runs from a Meckel's diverticulum. Then follows a long supra-duodenal
loop, drained partially by a " bridging " vein. Posterior to this is a typical supra-ctecal
kink, and then a pair of reduced caeca. The rectum is of moderate length and straight.

Fig. 27.

Intestinal Tract of Serpentarius reptilivoms. Lettering as before.

VTILTTJKID^E. — Neophron percnopterus (fig. 28) and other Vultures which I have
examined are closely similar. The duodenum is an irregularly expanded loop. Meckel's
tract presents three distinct minor loops (numbered 1, 2, and 3 in the figure), which
reappear constantly in Falconiform birds ; then follows an axial loop bearing Meckel's
diverticulum, and then several long, rather irregular loops, the last of which is a typical
supra-duodenal loop with " bridging " vein. The ceeca are vestigial, and above them lie
two supra-csRcal kinks drained by the rectal vein. The rectum is short and straight.

INTESTINAL TRACT OF B1EDS.

Fig. 28.

207

Intestinal Tract of Neophron percnopterus.
I, 2, 3, three anterior loops of Meckel's Tract. Other lettering as before.

(1) Gypaetincf. — Gypohierax angolemis (fig. 29) displays an alimentary tract extremely
like that of the true Vultures. The duodenum is simpler, being a long, narrow loop.
Meckel's tract hegins with three distinct minor loops (1, 2, and 3) ; then follows an axial
loop, with Meckel's diverticulum, and the remaining part of the tract, including the
supra-csecal kinks and the vestigial caeca, is precisely as in the Vultures.

(2) Polyborince. — In Polyborus brasiliensis (fig. 30), the Brazilian Caracara, the
duodenum forms a long, narrow loop, which is wound into a spiral. Meckel's tract
consists of a set of small loops arranged very symmetrically, the axial loop as usual
hearing a Meckel's diverticulum. There is a well-formed but single supra-caecal kink in
the normal position, and anterior to that a supra-duodenal loop. The caeca are vestigial,
and the rectum is short and straight.

(3) Accipitrinee. — In Circus c'meraceus (fig. 31) is to be found what may be regarded as
a central condition for the Falconidae. The duodenum is a large, very wide loop.

32*

208

DE. P. CHALMEES MITCHELL ON THE

Meckel's tract exhibits first three distinct minor loops, and then is prolonged axially,
bearing at its extremity a Meckel's diverticulum. The posterior part of the tract has one
or two minor loops and a large supra-duodenal loop with two " bridging " veins. Then
comes a large supra-caecal kink. The caeca are vestigial, and the rectum is short and
straight. Another Circus (the species of which was not identified), Clrcaetus gallicus,
and Helotarsus ecaudatus all exhibited precisely the same conformation.

Fig. 29.

Intestinal Tract of Gypohierax angolensis. Lettering as before.

(4) Aquilinee. — 1 have examined Aquila audax, A. chrysaetus, A. Verreauxi, Morphnus
guianensin* Spizactus coronatus, and jHaliaetus albicilla and H. leucogaster. I have
already described the conformation of the gut in the White-tailed Sea-Eagle (26. fig. 12).
The duodenum is very long, being thrown into a complicated system of secondary folds.
Meckel's tract, also, is much elongated and its greater part is composed of a series of
short irregular loops suspended at the periphery of an oval stretch of mesentery, the apex

INTESTINAL TRACT OF BIEDS.

209

Kg. 30.

Intestinal Tract of Polyborus braslliensig. Lettering as before.

Fig. 31.

Intestinal Tract of Circus fineraeeitg. Lettering as before.

210

DK. P. CHALMERS MITCHELL ON THE

of the system carrying a Meckel's diverticulum. There is a well-formed supra-duodenal
loop, which is spirally twisted, and a large supra-caecal kink has a similar arrangement-
The caeca are vestigial, and the rectum is short and straight. Haliaetus leucogaster
differs from the foregoing only in that its duodenum is spirally twisted. These two
birds are large and chiefly piscivorous, and the homoplastic increase in gut-length has
obscured the peculiar characters of the Falconiform gut, but the general symmetry, the
peculiar duodenums, and the well-formed supra-caecal kinks are sufficiently distinctive.
The other Aquilinae repeat exactly the pattern displayed by Circus, the only slight
modification I have found being that the supra-duodenal loop in Spizaetus, although
drained by a " bridging " vein, is very small. The supra-csecal kink is very large in
most and distinct in all.

(5) Buteonince. — Of these I have examined Astur tachiro, Asturiiia magnirostris,
Buteo erythronotus, B.ferox, B.jacal, Milvus govinda, M. ictinus, M. migrans. In all
the pattern of Circus is reproduced with close fidelity. The duodenum is irregular,
sometimes long, narrow, and straight as in the genus Buteo, sometimes irregularly
expanded, or folded upon itself several times as in some species of Milvus. Meckel's
tract is always as in Circus, save that in a Buteo no trace of Meckel's diverticulum was
retained. The supra-duodenal fold is always present as also is a large supra-caecal kink.
The caeca are vestigial and the rectum is short and straight.

(6) Falconince. — I have examined Falco concolor, F. Feldeggi, F. lanarius, F. mela-
nogenys, F. peregrinus, and Microhierax melanoleucus. The Falconinae certainly exhibit

Fig. 32.

•m

Intestinal Tract of Falco melanogenys.
«.»i., visceral nerve-chain. Lettering as before.

the most specialized or apocentric form of gut among the Falconida3. The duodenum is
always a large irregular loop, sometimes with minor folds (fig. 32), sometimes bent on
itself (fig. 33). Meckel's tract is always very much elongated in the axial line, and

INTESTINAL TEACT OF BIRDS. 211

invariably bears at its apex an unusually large Meckel's diverticulum. In many Falcons
the first portion of Meckel's tract displays the three minor loops which recur among
Pal coniform birds. In Falco melanogenys (fig. 32) and F. Feldeggi (fig. 33) two of these
have disappeared, probably in connection with the very great elongation of the tract as
a whole. The apical portion of Meckel's tract may be irregularly twisted as in the

Fig. 33.

Intestinal Tract of Falco Feldeggi. Lettering as before.

Peregrine Falcon and in Falco melanogenys (fig. 32), or it may be coiled into an
irregular spiral, an apocentric peculiarity found in many specialized types, for instance
in Pigeons and Passerines. There is always a supra-duodenal loop and a supra-caBcal
kink. The caeca are vestigial ; in a Peregrine Falcon I found only one present. The
rectum is short and straight.

3. — In the Osprey, Pandion haliaetus (fig. 31), the gut is enormously long,
and is of very narrow calibre, a modification obviously in association with piscivorous
habit. The duodenum is long and narrow. Meckel's tract is thrown into a very large
number of narrow loops, arranged round a nearly circular mesenteric expanse, the
diverticulum being in the usual place at the central point of the curved system. There
is no supra-duodenal loop, but there is a small supra-csecal kink. The caeca are vestigial
and the rectum is short and straight.

The Falconiformes are on the average rather large birds with diet in the main
carnivorous, with some exceptions which are piscivorous, and a few which live on
insects. Except in the piscivorous cases, there is little correction to be made for diet.

212

DE. P. CHALMERS MITCHELL ON THE

The carnivorous forms are on the average the larger, and the carnivorous shortening
of the gut is in consequence disguised by the relative increase in length associated with
size. The general features of the group are a tendency for the duodenum to he irregular,
enlarged, very long, or spirally twisted ; for Meckel's tract to exhibit three definite minor
loops anterior to a median loop bearing a Meckel's diverticulum, and, posteriorly to that,
first one or two irregular loops and then a supra-duodenal loop and at least one supra-
cjT-cal kink drained by the rectal vein ; the ca3ca are always vestigial and the rectum
short and straight. The departures from this common type are — first, the irregularities
in the fish-eaters ; second, a progressive tendency for the lengthening of Meckel's tract

Fig. 34.

Intestinal Tract of Pandion haliaetvg. Lettering as before.

in the axial line with consequent obliteration of one or more of the other minor loops,
and, in the most apocentric cases, with a spiral folding of the tract ; third, Serpentarius
shows distad of the first three loops of Meckel's tract a circular expansion of the mesentery
bearing a number of minor loops, and this condition leads naturally to the condition in
Cathartes, where the circular expansion involves the second and third of the definite
loops on the proximal side of Meckel's tract. Attempts have been made to show a more
intimate relation between the Cathartae and some of the Ciconiiform birds than between
these and other Falconiformes ; there is no ground for such a conclusion in the structure
of the intestinal tract. Still less ground is there for attempting to place in intimate

INTESTINAL TEACT OF BIRDS. 213

relation any of the Falconiformes with Cariama. The latter bird, as vvDl appear later,
is definitely a member of the Gruiform assemblage, and for relations between the
Gruifonn birds and the Falconiform birds it is necessary, so far as the characters of
the intestinal tract take us, to go back to the archecentric type underlying all birds.

SUMMARY OF THE PELARGO-COLYMBOMORPHINE BRIGADE. (Plate 21.)

At this point it is convenient to attempt a resume of the conclusions to which study
of the Intestinal Tract has so far led.

Taking the form in Palamedea as the archecentric type, it appears that the Struthious
birds are grouped indifferently around it, as they all display the archecentric character
in an unmodified or very slightly modified form. Of the Colymbomorphae, Colymbus
exhibits the simplest type, in fact a slight but definite modification of the Palamedea
form, consisting in the expansion of Meckel's tract into a set of straight, narrow loops,
one of which is axial and bears the diverticulum, the others being arranged nearly
symmetrically about the middle mesenteric vein. There is usually a supra-duodenal
loop ; the caeca are functional and the rectum is short and straight. This form is of
great importance, as it is a Pelargo-Colymbomorphine metacentre from which radiate the
type of intestinal tract displayed by Gadow's first Brigade of birds, including the Legion
Colymbomorphae with the Colymbiformes, the Sphenisciformes, and the Procellariiformes,
and the Legion Pelargomorphae with the Ciconiiformes, Anseriformes, and Ealconiform.es.
The Grebes are more apocentric modifications of this metacentric type, the change being
chiefly a reduction of the number of loops of Meckel's tract with a corresponding increase
in the length of the individual loops. A slight but still more apocentric modification of
the Pelargo-Colymbomorphine metacentre produces a new central position, which may
be called the Steganopod metacentre. The two chief additional characters of this are,
firstly, a tendency to complication and lengthening of the duodenum, a tendency which
in nearly every case is actually fulfilled; and secondly, the appearance immediately
above the caeca of a kink supplied by the rectal vessel, although belonging to the
drainage-area of the middle mesenteric vein. Meckel's tract does not differ funda-
mentally (fig. 35) from the condition in the Pelargo-Colymbomorphine metacentre, but
the caeca are reduced. The Sphenisciformes are modified from such a condition only by
the greater length of their gut and consequent increase in the number of minor loops
on Meckel's tract. The Procellariiformes are modified from it chiefly in the reduction
of the number of loops on Meckel's tract and the great increase in length of the
individual loops, the axial of which may be spirally twisted in the most apocentric
forms, e. g., the Oceanitidse. Of the Ciconiiformes, the Steganopods retain their meta-
centric position. The Ardeae start from that position (Scopus), but in the Ardeidae the
minor loops of Meckel's tract become very complicated and one of the two caeca is lost.
The Ciconii start from the Steganopod metacentre with forms like Anastomus, but they
rapidly reach a more apocentric condition, the chief peculiarity of which is the spiral
twisting not only of the duodenum but of the first minor loop of Meckel's tract, and
the twisting of these two spirals together, so that sometimes the blood-vessels are in

SECOND SERIES. — ZOOLOGY, VOL. VIII. 33

214

DE. P. CHALMERS MITCHELL ON THE

common. Platalea is less apocentric than the Steganopod metacentre. In it Meckel's
tract differs from that in the Pelargo-Colymbomorphine metacentre only in that it is
elongated, thrown into many short folds, and spirally twisted as a whole. Phcenicopterus
is like Platalea, but it is still more archecentric, inasmuch as its caeca are functional.
The Falconiformes have developed from the Steganopod metacentre so as to display a
central type of their own, characterized by an elongation of Meckel's tract, in the line
of the axial loop, the latter bearing the diverticulum, and the settling down of the gut

Falco

Fig. 35.

Ciconine Types Falcqninag Vulturida?

Polybonn.!
Buteoninffi

Ardeidas

Procellariiformes

Accipitrina
Aguilinae

Gypphierax Catharte?

\ x > - Serpentarius

Falconiform
Metacentre

Anastomus
Scopus

Sphenisci

Podicipedes

Colymbi

Spatula

Nettopus

Steganopod
Metacentre

' Pelargo-
Colymbimorphine
Metacentre

Anserine
Metacentre

Platalea

Phcenicopterus /

Anseranas

Mergus

Archecentric Types

Palamedea

Ratitae

Evolution of Intestinal Tract in the Pelargo-Colymbomorphine Brigade.
(For Colymbimorphine read Colymbomorphine.)

between this and the duodenum into three definitely formed loops. From this the higher
Palconidse move apocentrically, the axial loop of Meckel's tract increasing in length at
the expense of the others and being twisted into a spiral. G-ypohierax, with its irregular
minor loops on Meckel's tract and its two supra-csecal kinks, leads to the Vulturidae.
Serpentarius with its spherically expanded Meckel's tract arises somewhat outside the
Ealconif orm metacentre, and may well lead to the more apocentric Cathartse, which have

INTESTINAL TRACT OF BIRDS. '

215

completely lost the caeca. Pandion stands by itself; its gut is so extremely long that
none of the usual minor loops except the supra-caecal kink can be identified. Haliaetus
is a simple modification of the Falconiform metacentre, the irregularity being due to
increase in length. For the Anseridae it is necessary to go back to the Pelargo-Colymbo-
morphine metacentre. Anseranas is practically in that position unmodified, save that
the caeca are still longer, a condition common to all the Anseridae except some of the
Mergansers. The Cygnus type, with its three definite and contorted minor loops on
the anterior portion of Meckel's tract and its very long axial loop with peculiar blood-
vessels, gives an Anserine metacentre from which Nettopus and Spatula have diverged
still further. Mergus is probably a pseudocentric modification of the Spatula type.

TINAMIFORMES.

CRYPTTTRID^E. — Of these I have been able to examine the intestinal tract of several
specimens of Rhynchotus rufescens and Nothura maculosa. The conformation is prac-
tically identical in these two forms. The duodenum is a long narrow loop ; Meckel's
tract (Rhynchotus rufescens, fig. 36) is divided into two nearly equal parts, the large

Fig. 36.

Intestinal Tract of Rhynchotus rufescens.
c, compare loop similarly marked in Gruiformes, figs. 41, 42, 43, &c. Other lettering as before.

Meckel's diverticulum lying between the two. The first portion is one very long narrow
loop ; the second portion is an equally long and narrow supra-duodenal loop drained by a
branch of the middle mesenteric vein as well as by the usual " bridging " factor from the
duodenal vein. Meckel's diverticulum was very large in two specimens of Rhynchotus,
small in a third, and very small in Nothura. The small minor loop just distad of it

33*

216 DR. P. CHALMERS MITCHELL ON THE

represented in figure 36 was present in two specimens of Rhynchotus, absent in another,
and present in Nothura. The caeca were long in all, and Beddard (2) has notified the
presence of long caeca in the other genera and species, and of very peculiar caeca in
Calodromas. The rectum, is short and straight.

The figure may be taken as representing the morphological character of the Tract in
the Tinamus, as there is little correction to be made for habits, and as the conformation
is practically identical in relatively large forms such as Rhyncliotus, and relatively small
forms such as Nothura. It is clear, then, that the type of the intestinal tract in the
Tinamiformes is markedly apocentric, and differs from the archecentric type of Pala-
medea and of the Strutkious birds in that the rectum is straight, and more notably in
that Meckel's tract is expanded into two long, straight, and narrow loops, one anterior,
the other posterior to the rudiment of the yolk-sac, both being orthocrelous. Naturally,
as the Ratites have intestinal tracts of archecentric conformation, it is as possible that
the Tinamiform gut may be derived from the Ratite gut as from that of any other
archecentric form. But so far as the character of the intestinal tract goes, there is no
reason to associate the Tinamus specially with the Ratites. Nor is there any reason
to associate the Tinamus in this matter with the Gralliform.es, for in these the gut
shows a fundamental archecentricity underlying a tendency to apocentric development
in the direction of expansion of that part of Meckel's tract supplied by the recurrent
branch of the middle mesenteric vein, and there is no trace of this feature in the Tinamus.
The gut of the Tinamus bears a close resemblance to that of specialized Ralline forms
such as Otis (fig. 45). The gut of Otis, as I shall show later, can be regarded without
difficulty as an apocentric derivative of that of less specialized Ralline forms, and for
this reason the resemblance between it and the Tinamu may be merely superficial.
There is, however, another reason for not rejecting the Ralline affinities of the Tina-
miform gut too readily. As I shall explain later, I have only gone a little way in study
of the autonomic nervous system, and therefore cannot yet weigh the value of evidence
to be drawn from the modifications exhibited in different birds. There are, however, at
least two strongly contrasting types. In one of these the visceral nerve of Meckel's
tract forms a ganglionated chain which follows the contour of the gut. This type
occurs in Palamedea (fig. 1, v.n.}, in the Struthious birds, and in the GJ-alliformes. In
the other type there are a much smaller number of larger ganglia on the visceral nerve
(Otis, fig. 45, v.n.). This type occurs in some of the Ralline birds and in at least
Nothura. Another striking resemblance is that between the conformation of the gut
in the Tinamus and in Opisthocomus. To this I shall recur later. The conformation
of the gut, then, supports those who, like Gadow, isolate the Tinamus as Tinamiformes,
but at the same time suggests possible relationships with Gfruiform birds and with
Opisthocomm.

GALLIFORMES.

MESITES. — I have no information regarding this group. Naturally knowledge of the
conformation of the alimentary tract is much to be desired in the case of a bird the
affinities of which are doubtful.

INTESTINAL TEACT OP BIRDS.

217

TURNICES. — In the Turnices (I have .not seen Pedionomus) the duodenum (Turnix
Dussumieri, fig. 37) is a long, narrow loop ; Meckel's tract is suspended at the periphery
of a nearly circular mesenteric expansion and bears no trace of the diverticulum.
There is no specially formed supra-duodenal loop, but the very large cseca (which are
dilated towards their extremities) are drained partly by a bridging vein from the
duodenal vein. The rectum is relatively long and very slightly expanded into folds.

Fig. 37.

Intestinal Tract of Turnix Dussumieri.
r., recurrent factor of middle mesenteric vein. Other lettering as before.

This intestinal tract is markedly archecentric. The simple character and the arrange-
ment of the blood-vessels are closely similar to the condition of Palamedea. The chief
modifications are absence of the Meckel's diverticulum, the increased length of the caeca,
and the relatively shorter rectum. So far as the character of the gut indicates systematic
position, there is no reason either to retain Turnix among the Galliformes or to remove
it from that assemblage. It is simply an archecentric form.

218

DE. P. CHALMEES MITCHELL ON THE
G A L L I.

MEGAPODID-S:. — In Talegallus Lathami (fig. 38) the duodenum is a straight, narrow
loop of moderate length. Meckel's tract is swung at the periphery of an expanse of the
mesentery, and forms almost a complete circle of which the middle mesenteric vein,
running from the Meckel's diverticulutn, forms a diameter ; for the greater part of its

Fig. 38.

Intestinal Tract of Talegallus Lathami. Lettering as before.

course this tract is thrown into very regular corrugations, but its distal portion is nearly
straight. The recurrent factor of the middle mesenteric vein supplies a considerable part
of Meckel's tract. The caeca are of moderate length and are drained partly by a
" bridging " vein, and the rectum is fairly long and very slightly thrown into folds. A
striking feature in Talegallus, as in other Galli, is the conspicuous visceral nerve (v.n.),
which in this bird is a ganglionated chain following the sweep of Meckel's tract.

CEACID^E. — In Crax Daubentoni (fig. 39) the only notable difference from Talegallus
in the formation of the gut relates to Meckel's tract. This area is much elongated
distally, with the result that the recurrent factor of the middle mesenteric vein (r.) which
supplies the distal portion of the tract is much enlarged, and appears to be the direct
continuation of the middle mesenteric. That vein, however, in the embryo runs out as
usual to the diverticulum as I figured in the case of Argus giganteus (26. fig. 2), and its
real termination in the adult is one of the minor branches of the main blood-channel.

INTESTINAL TEACT OF BIRDS.

219

In association with the growth of the posterior portion of Meckel's tract, the visceral
ganglionated chain forms a curious elongated loop, which, on comparison of figs. 38
and 39, will be seen to be obviously connected with the distal growth of the tract. In
two other species of Crax and in two species of Penelope the conformation of the gut
with its blood-vessels and nerve was identical in all main points with the condition just
described.

Fig. 39

m—

Intestinal Tract of Crax Daubentoni. Lettering as before.

GALLID^E. — I have examined a considerable number of these, including species of
Argus, Caccabis, Callipepla, Coturnix, Francolinus, Gallus, Lophophorus, Pavo,
Phasianm, Perdix, and Tetrao, and in all the conformation closely resembles that
shown in the figure of Pavo cristatus (fig. 40). Meckel's tract is more elongated in
the axial line than in the Cracidse, and the middle mesenteric vein runs obviously from
the remains of the diverticulum. There is a similar distal prolongation of the tract
drained by a large recurrent vein, and to the straight distal portion of this the very

220

UK. P. CHALMEES MITCHELL ON THE

large caeca are closely applied, and are drained partly by the recurrent vein and partly
by a " bridging " vein. The rectum is relatively short and straight.

In the G-alli generally the only homoplastic cause of apocentricity that has to be
allowed for is the increased length in the larger forms, and this is not sufficiently great
to distort the morphological pattern of the intestinal coils. In all, the general character
is markedly archecentric ; in the Megapodidse the archecentricity is most definite. In
the Cracidae there is an apocentric modification of a definite nature, although not great,
and consisting in the peculiar expansion of the distal region of Meckel's tract with

Fig. 40.

Intestinal Tract of Pavo cristatv*. Lettering as before.

coincident alteration of the visceral uerve and of the factors of the middle mesenteric
vein. In the Gallidae a similar change has occurred, but has affected the blood-vessels
and the visceral nerve (the course of the latter, not indicated in the figure, very closely
resembles that in Talegallus) to a much smaller amount. On the other hand, the caeca
in the Gallidae are much larger, and the small factor of the duodenal vein, which in Grax
runs from them within the mesentery, is enlarged in the Gallidae and has broken through
the mesentery. It is plain that, so far as the conformation of the intestine indicates, the
Galli are all closely allied, but there is a distinct basis for their division into Megapodidaa,
Cracidae, and Gallidae.

OPISTHOCOMI.

I have already described and figured the intestinal tract of Opisthocomm cristatus
(27. fig. 1). The duodenal loop is short and wide. Meckel's tract is thrown into three
well-marked narrow loops, the second of which is rather wider and has a tendency to be
twisted into a very slight spiral. In a chick and in two adults I found no trace of

INTESTINAL TKACT OF BIEDS. 221

MeckePs diverticulum, but towards the extremity of this loop the place of attachment
of the yolk-sac was marked by a strong remnant of a ventral mesentery. In a third
adult this mesentery ran to a very small vestige of Meckel's diverticulum. The most
distal loop of Meckel's tract was wide, and closely applied to it were the pair of large
caeca, drained partly by a "bridging" factor of the duodenal vein. The rectum is
unusually long, retaining the archecentric condition of being thrown into minor folds.

It is plain that the condition of the intestinal tract in this group is markedly apocentric,
except with regard to the rectum. From this point of view the Opisthocomi do not find
a natural place among the Galliformes — first, because in these latter the general condition
of the gut is archecentric ; and second, and more important, the small degree of apocen-
tricity displayed among some of them consists of a peculiar expansion of the distal
portion of Meckel's tract, and there is no trace of this feature in Opisthocomm. The
apocentricity of Opisthocomus may be compared first with that of the Tinamus. If
the very small loop seen just distal to Meckel's diverticulum in Rhynchotus (fig. 36) were
prolonged, carrying with it the diverticulum, the condition in Opisthocomus would be
reached. A second suggestion of relationship is, as I have already pointed out, given by
comparison with the form of the tract in Pterocles and the Pigeons. In Pterocles
(26. fig. 18) the condition of Meckel's tract and of the long caeca is extremely like that in
Opisthocomus, the most notable difference being that in Opisthocomus the axial loop bearing
the diverticulum is slightly twisted. In the Pigeons (26. fig. 19) the twisting of the axial
loop may be carried much further and the casca are reduced. When I first made this
comparison of the gut in these three groups, I accepted the common view that eutaxy or
quintocubitalism was a primitive condition, and diastaxy or aquintocubitalism a derived
condition, and it was with hesitation that I put Opisthocomus, an eutaxic form, between
Pterocles, a diastataxic form, and the Columbidse, then believed to be diastataxic. Since
then I have shown that the eutaxic condition is probably a multiradial apocentricity
derived independently from the diastataxic, archecentric condition of the wing. Moreover,
I have shown (28) that among the Columbidae eutaxic forms occur. There is therefore
no difficulty in the way of supposing that the gut forms in the Pigeons and in Opistho-
comus are derivatives of the condition in Pterocles. In the paper in which I described
the intestinal tract of Opisthocomus (27) I pointed out also that there were resemblances
between the gut of Opisthocomus and the gut of the Cuculidae, thereby recalling Garrod's
(13) suggested relationship between Fowls, Opisthocomus, and Cueulidae. It is true that
among the Cuculidse Meckel's tract is thrown into three loops, but an examination of a
larger number of Cuculidse has shown me that important differences distinguish the three
loops in Opisthocomus and the loops in Cuculidas.

Taking the Galliformes as a whole, it appears that the form of the gut in the Turnices
is archecentric; in the Galli it is still archecentric, but with a tendency to a special mode
of apocentricity ; in the Opisthocomi it is markedly apocentric, and the apocentricity is
quite different in kind from that found among the Galli, but with marked resemblances
to the condition in Pterocles and the Pigeons.

SECOND SEBIES. — ZOOLOGY, VOL. VIII. 34

222

DK. P. CHALMERS MITCHELL ON THE

GRUIFORMES.

. — In Ocydromus australis (fig. 41) what I find to be the typical Ralline
conformation of the intestinal tract is presented. The duodenum is a straight, narrow
loop of moderate length. Meckel's tract is drawn out into a definite number of loops,
all of which are fairly straight. The first of these, marked " «" in the figures, succeeds
the duodenum ; the second (" b " in the figures) is axial and bears on its distal side a
large Meckel's diverticulum. The third, marked " c," like the others is a narrow loop
belonging to the drainage of the middle mesenteric vein ; and the fourth is a well-marked
supra- duodenal loop drained by more than one bridging factor of the duodenal vein. The

Fig. 41.

ex.

-OC.2.

L-OC.I.

-S.DT.

Intestinal Tract of Ocydromug australis.
a, 6, c, the three loops of Meckel's Tract characteristic of the Gruiformes. Other lettering as before.

long caeca are closely attached to this last loop of Meckel's tract. The rectum is straight
and of moderate length. This conformation is found in all the members of the Rallidaj
that I have examined, e. g., Aramides ypecaha, Crex pratensis, Oallinula phcenicura,
lonornis marfinicus, Porphyrio cceruleus, and P. poliocephalvs. I described this condition
of the gut correctly in a former paper, but in the drawing of Crex (26. fig. 13) the third
minor loop has been omitted by an unfortunate mistake, so that the description does not

INTESTINAL THACT OF BIRDS.

223

tally with the figure. The differences which occur among the Rallidae are insignificant ;
in some, there are two " bridging " veins, in some, one ; the Meckel's diverticulum, which
is always large, may be extremely large, the size of this structure being a characteristic
feature of the group ; the third svibsidiary loop of Meckel's tract occasionally is relatively
smaller than the other loops.

(1) Gruina>. — In Grus viryo (fig. 42) and other species of Grus, and species of
Anthropoidcs and Balearica are practically identical, the large size has brought with
it a relatively great increase of length, but the fundamental identity of the conformation
of the gut with that exhibited by the Bails is obvious. The duodenum is similar,

Fig. 42.

Intestinal Tract of Grus virgo. Lettering as in fig. 41.

although, on account of its great length, it is partly bent. Meckel's tract displays the
same four loops : " « " being widened out ; " b" the axial loop being very lonij but carrying
a Meckel's diverticulum in the characteristic position, on the distal limb of the loop ;
" c" being small but compound; and the supra-duodenal loop to which the long caeca
are attached being much enlarged. It is plain that wre have here a simple modification
of the Balline pattern, the modification being due to the relatively greater length of gut
that occurs in larger birds.

(2) Aramince.- — The very interesting form Aramus scolopaceua (fig. 43) displays a
conformation of the intestinal tract which differs only in minor respects from that in the

34*

221

DE. P. CHALMERS MITCHELL ON THE

Rails and Cranes. The duodenum is normal ; Meckel's tract displays the loops which I
have marked "a" and "6" precisely as in the Rails, and "b" the axial loop bears
a large diverticulum on its distal limb. The posterior portion of the tract differs : in
place of " c" the third loop, and of the normal supra-duodenal loop, there are a set
of irregular small loops. The caeca are short but wide and in the natural condition
contain faecal matter. The rectum is short, wide, and straight. I am disposed to think
that the type iu.Aram.u8 is more archecentric than the types displayed by the Cranes and
Rails. Although the axial loop with the diverticulum on its lower limb is character-
istically Ralline, the general conformation of Meckel's tract is much more like the

Fig. 43.

YU--

Intestinal Tract of Aramus scnlopaeeus. Lettering as in fig. 41.

archeceutric condition, and this similarity is increased by the moderate length of the
caeca and the absence of a specialized supra-duodenal loop. I think it is more probable
that the long caeca of the Cranes and Rails are an apocentric modification than that the
relatively shorter caeca of Aramus are pseudocentric degenerations, for, as I hope to show
later, in the vast majority of cases where the cseoa are obviously degenerate, the
" bridging " factor or factors from the duodenal vein, which originally drained them,
persist to drain a specialized supra-duodenal loop, and there is no trace of this in
Aramus.

INTESTINAL TEACT OF BIEDS.

225

(3) Psophiince. — As the position of Psophia is one concerning which there has been
DO little difference of opinion, it is interesting to find that the conformation of the gut
in Psophia crepitans (fig. 44) is typically Ralline. As will be seen, it conforms in every
way to the character typical of the Kallidse. As it is probable that the differences
between the type in the Cranes and that in the Rails is due merely to the larger size of
the former, there is no reason to be drawn from the character of the gut for placing
Psophia preferentially either with the Cranes or with the Rails. It displays the ground-
form common to both sets. Psophia obscura is simplified in the direction of Cariama
and the Bustard ; the loops on either side of the axial loop, i. e. " a " and " c," have
practically disappeared, so that the conformation in Otis is closely imitated.

Fig. 44.

Intestinal Tract of Psophia crepitans. Lettering as in fig. 41.

. — I have already described and figured the intestinal tract in Cariama
cristata (26. fig. 14), and the condition in Burmeister's Cariama (Chunga Bnrmeisteri) is
practically identical. The duodenum is a long narrow loop. Meckel's tract displays
the Ralline loops " a " and " b " ; but the two are merged proximally, " b " displaying a
Meckel's diverticulum on the distal limb. There is no trace of loop " c " ; but there is a
large supra-duodenal loop drained partly by a "bridging" vein, and having the long
caeca closely applied to it. The rectum is straight but of fair length. It is plain
that the intestinal tract of the Dicholophidae presents an apocentric character of a
definite nature, but which may be regarded as a simple derivative of the type shown in

226:

DR. P. CHALMERS MITCHELL ON THE

the Rallidse and Gruidse, the loops of Meckel's tract being long and straight, the first
and second being partly fused and the third having disappeared.

OxiDiDyE. — I have had the opportunity of examining only Otis tarda, the Great
Bustard. In it (fig. 45) the duodenum is a long narrow loop. Meckel's tract is thrown
only into two loops, of which the second is a very large supra-duodenal loop drained by
a pair of large bridging vessels, and has Jong and peculiar caeca closely applied to it.
The proximal loop 1 take to be the axial loop seen in the Rallidae ; it bears upon it, low

Fig. 45.

un

S.DF

PC ^

Intestinal Tract of Otis tarda.

n.i?., nerves entering mesentery; v.ii.u:., ganglion iu duodenum from which nerves pass to eseca and supra-
duodenal loop; v.n.g., large gnngliou of visceral nerve; v.n., branch of visceral nerve in rectum.

down on its distal limb, a Meckel's diverticulum. A pair of very small but quite
definite folds, marked "a" and "c" in the figure, I take to represent the similarly
designated large loops in the Rallidae and Gruidse.

The intestinal tract of Otis is certainly markedly apocentric. The loops are all very
definite and the supra-duodenal loop in particular is highly specialized, being closely
applied to the duodenum, over which it lies in the unfolded condition, and from which
it receives not only a pair of bridging veins, but two branches of the autonomic visceral

INTESTINAL TRACT OF BIKDS.

227

nerve coining from a special ganglion in the duodenal loop (fig. 45, v.n.x.). The peculiar
caeca which have been described by Bcddard (2. p. 332) are specialized not only in their
great length but in their internal structure. The general character of the gut, however,
especially if I am correct in my interpretation of the small folds marked " a " and " c,"
is obviously Ralline. The resemblance between the guts of Otis and of the Tinamus is
certainly striking, the only notable difference being that in the Tinamu the cseca are
less specialized and that Meckel's diverticulum lies distad of the great loop of Meckel's
tract rather than actually on it. I regard the closeness of the resemblance as not
definitely morphological.

.*. — In Ithinochetus jubatus, the Kagu (fig. 46), the duodenum is

Fig. 46.

Intestinal Traot of Kliinuchetut jubatus. Lettering as in fig. 41.

a simple, short, and narrow loop. Meckel's tract shows first a pair of minor loops, the
second of which is double, then an axial loop with a large diverticulum about the middle
of its distal limb, then a large simple loop marked " <?," then a long narrow supra-
duodenal loop with a bridging vein. The cseca are rather short, and the rectum is
straight, wide, and of moderate length. The intestinal tract of this bird certainly differs
considerably from that of other members of the Gruiform assemblage. The most
important differences are the presence of an additional loop on Meckel's tract, the
.•circumstance that the whole of that tract, with the exception of the supra-duodenal

228

DE. P. CHALMEES MITCHELL ON THE

loop, displays in the unfolded condition a slightly spiral twist, and the shortening of the
caeca, a character shared with Eurypyga. The resemblance to the Ralline type, how-
ever, is fairly strong. Meckel's tract is produced into a set of minor loops, of which
two represent " a," while " 5," with the diverticulum midway on the distal limb, and " c "
are normal, and the last is a supra-duodenal loop simplified in correspondence with the
reduction of the cseca. Rhinochetus has been compared with Scopus, but the character
of the tract does not support this comparison. In RJiinochetus there is no trace of the
twisted duodenum, the diverticulum is in the Gruiform position, and Meckel's tract is
thrown into definite minor loops.

EURYPYGED.E. — Eurypyga helias (fig. 47) departs still more from the other Gruiformes.

Fig. 47.

Intestinal Tract of Eurypyga helias. Lettering as before.

The duodenum resembles that of the other forms, but Meckel's tract is much more
archecentric, the greater part of it consisting of an irregularly folded gut swung at the
periphery of an oval expanse of mesentery. The large diverticulum, however, is not at
the apex of the tract, but in the typical Gruiform position on the distal limb of the axial
loop. The distal portion of MeckeFs tract is a definite supra-duodenal loop with
bridging vein. The caeca are vestigial, and the rectum is long and nearly straight.

HEMORNlTHnxE.— In Heliornis fulica (fig. 48) the typical Ralline or Gruiform
characters again appear. The duodenum is simple and narrow. Meckel's tract displays

INTESTINAL TRACT OF BIEDS.

229

the three loops " a" " b," and " c " very definitely drawn out. In H. surinamensis, the
conformation is similar, although the loops are not so independent. In neither is there
a diverticulum present. The posterior portion of Meckel's tract is a definite supra-
duodenal loop drained by two " bridging " vessels. The caeca are of moderate length, and
the rectum is as in the others.

It is plain that what I term a metacentral condition underlies the conformation of the
alimentary tract in the Gruiform assemblage. This metacentre possesses a simple
duodenum ; Meckel's tract is produced into four straight loops, of which " J," the axial

Fig. 48.

Intestinal Tract of Heliornis fulica. Lettering as in fig. 41.

loop, carries the diverticulum on its distal limb, and of which the last is a large supra-
duodenal loop drained by bridging vessels and with the long caeca closely attached to it.
The Rallidse and Gruidse display this metacentral character without modification of any
important kind. The Dicholophidse and the Otididse have moved apocentrically from it
by reduction of loops " a " and " c " and greater specialization of the supra-duodenal
loop. Heliornis is metacentral. RMnochetus and Eurypyga are slightly modified, the
apparent archecentricity of the latter being possibly pseudocentric.

SECOND SERIES. — ZOOLOGY, VOL. VIII.

35

230

DK. P. CHALMEES MITCHELL ON THE

CHARADRIIFOllMES.

LlMICOL^E.

K. — I Lave already described and figured the conformation of the intes-
tinal tract in the Curlew (Numenius arquata, 26. fig. 15). The duodenum is straight
and narrow. Meckel's tract is more archecentric than in the Gruiformes, inasmuch as
the minor loops are not well separated from the general course of the gut. None the
less, such minor loops exist, and markedly recall the common Gruiform type, although
the condition is less apocentric. There are three main loops corresponding to " «," " b,"
and " c " of the Gruiform gut, and, as in that assemblage, " b " the axial loop bears on
its distal limb a very large Meckel's diverticulum. The distal part of the Tract consists

Fig. 49.

Intestinal Tract of Tringa alpina. Lettering as in ing. 41.

of a supra-duodenal loop not well separated from the general outline of the Tract, but
drained by a " bridging" vein, and having closely attached to it the pair of long ca^ca.
The rectum is rather short and straight. The condition in Himantopus is similar to this
but still more archecentric — that is to say, the loops are still less marked off from the
general sweep of Meckel's tract. Vanellus vulg aris and V. cayennensis are like Numenius.
In Tringa alpina (fig. 49) a simple modifica tion of the Numenius condition is presented.
Loop " b " of Meckel's tract has grown out axially, leaving the large diverticulum at its

INTESTINAL TRACT OF BIEDS.

231

base, and the supra-duodenal loop is better separated. In Recur cirostra avncetta a still
more apocentric modification in the same direction is reached. The axial loop has
grown outwards to more than twice the length attained in Tringa, and is folded irregu-
larly as a flat ribbon, partly rolled on itself and partly spirally twisted. The caeca
remain long, and the other parts of the gut are as in Tringa. A still greater apocentric
divergence of precisely the same kind occurs in Scolopax rusticola (Plate 22). I have
already figured the very remarkable conformation assumed by its gut (26. fig. 16). The

Fig. 50.

Intestinal Tract of Chionis alba. Lettering as in fig. 41.

change is simply that the axial loop has grown still longer than in JLecurvirostra, and
the rolling-up and spiral twist is still better marked. The caeca have degenerated, and
have left isolated a small supra- duodenal loop with a bridging vein.

CHIONID^:. — Chionis alba (fig. 50) shows a comparatively slight modification of the
Numenius type, and is plainly much more archecentric than most of the Charadriidae.
Mecke!' s tract is relatively longer ; the axial fold with its large Meckel's diverticulum is

35*

232

DE. P. CHALMERS MITCHELL ON THE

Fig. 51.

Intestinal Tract of Glareola ondaris. Lettering as in fig. 41.

Fig. 52,

Intestinal Tract of Thinocorys rumicivorus. Lettering as in fig. 41.

INTESTINAL TRACT OF BIRDS.

233

practically identical with that of Numenius ; but the fold " a " is represented by two
loops, and loop " c " is partly fused with, or not separated from, the very large supra-
duodenal loop to which the long caeca are attached.

GLAREOLIDJE. — In Glareola ocularis (fig. 51), and G. pratincola is practically identical
in this matter, another very simple modification of the Numenius type is displayed.
The duodenum is similar ; Meckel's tract is relatively shorter, and is thrown into loops
" «," " b " with a small Meckel's diverticulum at its base, and a large and wide supra-
duodenal loop, loop " c " not being formed. The caeca are attached in the usual way to
the supra-duodenal loop, and the rectum is short, wide, and straight.

Fig. 53.

m

/ft

Intestinal Tract of Parra jacana. Lettering as in fig. 41.

. — In Thinocorys rumicivorus (fig. 52) a condition essentially similar to
that found in Glareola is displayed. The duodenum is similar ; Meckel's tract is
suspended at the end of an oval stretch of mesentery, and in it loop " b " with the
diverticulum at its base is obvious, although in the drawing it is represented as turned
up instead of running out in the axial line. Loops " a " and " c" are not differentiated,
and the supra-duodenal loop to which the long caeca are attached is not sharply
separated off except at its apex. The rectum is straight, wide, and of moderate length.

(EDICNEMIDJE. — In (Edicnemus scolopax, the Stone-Curlew, there is displayed a con-
formation closely similar to that found in Thinocorys and Glareola. The duodenum

234

DE. P. CHALMERS MITCHELL ON THE

is straight and simple, although rather long. Meckel's tract displays a small loop " «,"
then an axial loop " b " with Meckel's diverticulum on its distal limb ; loop " c" is not
differentiated from the supra-duodenal loop, to which the long cseca are attached. The
rectum is straight. It is plain that (Edicnemus is more archecentric than the ordinary
Curlews, and that its nearest allies are the Glareolidae, Parridse, and Thinocoridse.

^. — In Parrajacana (fig. 53, p. 233) the same ground-form is ohvious, although

Fig. 54.

Intestinal Tract of Par/vphila eburnea. Lettering as before.

there is a good deal of individual specialization. The duodenum is similar. Meckel's
tract exhibits loops "a" and " b," the latter with the diverticulum at its base. There is
no trace of loop " c," the distal portion of Meckel's tract appearing as a single very
large supra-duodenal loop supplied by two " bridging " veins and with the proximal
portion of the gut unusually dilated. The cseca are vestigial, and the rectum is short and
straight. Except for the vestigial cseca, Parra is very like (Edicnemus.

INTESTINAL TEACT OF B1EDS.

235

It is plain that in the Limicolse there is displayed a ground-form very close to what I
described as the Gruiform metacentre, but rather more archecentric than that. Prom
this condition the Charadriidso exhibit a striking set of progressively more apocentric
modifications, reaching a climax in the conformation of the gut in Scolopax. The other

Fig. 55.

Intestinal Tract of Stercorarius crepidatus. Lettering as before.

families, the Chionidae, Glareolidse, Thinocoridae, (Edicnemidae, and Parridtc, all display
types closely allied to that of Numenius, but still simpler. In other words, a form of
gut like that in Numenius is a metacentre common to the Gruiformes and the Limicolse :
from this the Gruiformes have diverged in one direction, the specialized Charadriidte in
another, while the other families cluster about the metacentre.

230

DE. P. CHALMERS MITCHELL ON THE

LA RI.

LARIDJE. — I have already described and figured the conformation of the gut in Larus
marinus (26. fig. 17). The duodenum is narrow and straight. Meckel's tract is thrown
into a series of irregular loops, the first of which prohably represents " «," while " b "
appears as a number of closely-set short loops, proximad of the diverticulum. " c " and
the large supra-duodenal loop are both present. The caeca are vestigial, and the rectum

Fig. 56.

Intestinal Tract of Sterna hirundo.

is rather long. Other specimens of Larus marinus were identical, except that Meckel's
diverticulum was absent, a condition frequent in the Gulls. Larus argentatus was
closely similar, and L. ridibundus differed only in that the loops of Meckel's tract were
more definite and were twisted into a common spiral. In Pagophila eburnea (fig. 54,
p. 234) the general conformation is similar. Loop "c" and the supra-duodenal fold are both

INTESTINAL TEACT OF BIEDS.

237

larger. However, the most striking circumstance is the appearance above the caeca in
the posterior part of Meckel's tract of a set of kinks drained by the rectal vein. In the
drawing, the relative importance of these has been slightly exaggerated ; but I was
anxious to call attention to them, as their presence suggests either an unlikely affinity
between the Laridso (and the Alcidse) and the Pelargo-Colymbomorphine assemblage, or,
more probably, that the existence of this kink is a multiradial apocentricity to which too
much importance must not be attached. In Stercorarius crepidatus (fig. 55, p. 235) a

Fig. 57.

Intestinal Tract of Fratercula arctica. Lettering as in fig. 41.

more archecentric type of gut is displayed, the arrangement rather closely resembling
that found in Thinocorys and Glareola. The duodenum is simple. Meckel's tract exhibits
two loops proximad of the diverticulum, while distad of it is a very wide area repre-
senting a supra-duodenal fold and an undifferentiated loop " c." The caeca are long and
the rectum is straight. In Sterna hirundo (fig. 56) the conformation is more apocentric,
and the pattern resembles that in many of the specialized Gruiform and Charadriiform
types. Meckel's tract is thrown into three minor loops, which, however, are better left

SECOND SERIES. — ZOOLOGY, VOL. VIII. 36

238

DR. P. CHALMEES MITCHELL ON THE

unidentified, as the diverticulum is not present, and there is a highly specialized supra-
duodenal loop left free by the degeneration of the caeca and closely resembling the
duodenum in shape. This similar moulding of the duodenum and the supra-duodenal
loop is a condition found in many of the higher forms with relatively short guts.

ALCID^E. — In Fratercula arctica (fig. 57) a conformation of the same general character
as that in many of the Gulls appears. The duodenum is of moderate size and not
twisted. The greater part of Meckel's tract consists of a set of short loops, the axial
of. which bears a large diverticulum in the Ralline position, and of a very large supra-

Fig. 58.

Intestinal Tract of Lomvia troile. Lettering as before.

duodenal loop from which there has not been separated loop " c." Above the vestigial
caeca is the curious supra-caecal kink to which attention was drawn in certain Gulls.
In Lomvia troile (fig. 58) a Gull-like pattern is displayed. There is no diverticulum to
indicate the exact topography. There is a well-formed supra-duodenal loop and a large
supra-csecal kink. The caeca are vestigial, and the rectum is extremely short.

The Laridae and Alcidse are families of birds typically fish-eating, and in which
lengthening and narrowing of the gut has to be allowed for. That allowance having

INTESTINAL TKACT OF BIEDS, 239

been made, the various conformations exhibited by the different genera of both families
(from the point of view of conformation of the gut no distinction can be made between
the families) appear obviously to belong to the simpler E/alline and Charadriiform
patterns. The only other possible suggestion of affinity is provided by the existence of
the supra-csecal kinks in some Gulls and Hawks ; but, as the other portions of the gut
are so unlike the conformations found among the Pelargo-Colymbomorphine assemblage,
I think the presence of the kinks in these few forms must be taken as a multiradial
modification.

Pteroclo- Columbine Assemblage.

PTEROCLES. — I have already described and figured the gut in Pterocles bid-net us (26.
fig. 19). The duodenum is straight and rather narrow. Meckel's tract is expanded to
form three definite loops. The first of these is long, expanded at its apex, and curiously
bent on itself. The second is an axial loop bearing Meckel's diverticulum at its apex.
The third is a definite supra-duodenal loop, drained by a " bridging " vein and having
the long ca?ca closely applied to it. The rectum is short and straight. In Plerocles
alchata the condition is exactly similar, except that the ca?ca are relatively longer.

C o L TJ M B JE.

I have examined a large number of Columbidse, including the genera Caloenas,
Chalcopelia, Chalcophaps, Columba (many species and varieties), Columbula, Geopelia,
Goura, Leucosarcia, Ocyphaps, (Ena, Phaps, Phlogoenas, Ptilopus, Starnoenas, Treron,
and Turtur. The conformation of the gut is best understood by taking it as a simple
but more apocentric derivative of the condition in Pterocles. The duodenum is a
simple loop. Meckel's tract is thrown into the same three folds. The first is simpler
than in Pterocles. The second or axial loop is usually very much longer, and it may be
twisted into an elaborate spiral (26. fig. 19), and as in Pterocles bears the diverticulum,
or a very small vestige of it marked by a ventral mesentery, or no vestige at all.
The third loop is a definite supra-duodenal loop very closely modelled on the duodenum,
and drained by a " bridging " vein. The ca3ca are always vestigial and not infrequently
are absent. Beddard (2. p. 308) mentions that in an example of Tympanistria bicolor
only one vestige was present. I have found similar variations not infrequently in the
varieties of the genus Columba, but apparently only as individual variations. The rectum
is always very short and straight.

There is little difference in the conformation of the gut in the different genera and
species. The larger forms in most cases have the length of gut relatively greatly
increased, and the increase is most noticeable in the spirally twisted axial loop. In
some small forms, such as (Ena, the gut is absolutely and relatively shorter, and there
is little trace of the spiral. Precisely a similar state of affairs exists among Passerines,
where size is one of the chief factors in determining the length of the whole gut, and the
consequent elaboration of the axial spiral. In Ptilopus and other fruit-eating Pigeons
the whole gut is extremely reduced in length and is very wide. In such cases the
relation of the pattern to that in ordinary Pigeons can only be guessed.

36*

DE. P. CHALMEES MITCHELL ON THE

From the point of view of conformation of the intestinal tract, it does not appear that
the Pteroclo-Columbae stand in close relation to other Charadriiform birds. The only
character in common is the tendency for Meckel's tract to be drawn out into long minor
loops, and the particular apocentricity which reaches its climax in the elaborate spiral of
the axial loop of Pigeons is different from that in the specialized Charadriidae such as
Recurvirostra and Scolopax, the difference being made plain by the relative positions of
Meckel's diverticulum. The comparison with Opisthocomus seems to give a more fertile
suggestion.

SUMMARY OF THE ALECTOROMORPHINE LEGION. (Plate 22.)

Gadow unites the Tinamiformes, Galliformes, Gruiformes, and Charadriiformes into
the Alectoromorphae, the first Legion of his second Brigade, and I propose now to review
shortly the characters of the intestinal tract presented in this large assemblage. It must
be clear that the conformations of the gut within this group do not follow many of the
more generally accepted modes of subdividing the group so readily and naturally as in
the case of the Pelargo-Colymbomorphine Brigade. Those, I think, who have paid most
attention to the classification of birds will be prepared for this result, for the many and
different divisions proposed suggest that the assemblage is still in indifferent order.
Prom the point of view of the subject of this Memoir, it is in the first place clear that
the Turnices and Galli stand apart from the other groups, inasmuch as their pattern of
intestinal tract has remained markedly archecentric, differing extremely little from that
in Palamedea. Next, the Tinamidse retain no suggestion of Struthious affinity ; they
have moved far apocentrically, while the Ratites, like the Galli, have remained close to
the archecentric condition. The Tinamidae present on the one hand most striking
resemblances with the Otididae, an apocentric type of the Gruiformes. On the other
hand, the radius of apocentricity on which they lie suggests that of Opisthocomus,
Pterocles, and the Columbae. Opisthocomus, one of the Galliformes of Gadow, certainly
stands far apart from the others, and, so far as the gut is concerned, an affinity with
Pterocles and the Columbse is most clearly indicated. The guts of Columbae and of
Pterocles are extremely similar, and there is no indication of any affinity with the
conformations exhibited in the other Charadriiformes or Gruiformes.

The Galli, Turnices, Opisthocomus^ and Pteroclo-Columbae having been disposed of,
there is less difficulty in arranging the conformations of the gut in other members
of the Alectoroinorphine Legion in coherent order. A conformation that is a simple
derivative of the archecentric form appears to underlie all of them, and this may be
called a Limicoline-Gruiform metacentre. The duodenum is simple ; Meckel's tract is
slightly elongated, but the elongation is proximad of the diverticulum so that that
appears on the distal limb of an axial loop. The distal region of Meckel's tract shows
the beginning of separation into a definite supra-duodenal loop to which the long caeca
are attached, and into a loop between that and the diverticulum. The rectum is short
and straight. The simpler Limicoline families, such as the Chionidas, Glareolidae,
Thinocorida?, and Parridae, are extremely close to this Limicoline-Gruiform metacentre ;
and the Lari, which from this point of view present no clear distinction into Alcidae and

INTESTINAL TRACT OF BIRDS.

241

Laridae, are derivatives of the pattern only slightly modified by increased length. A
modification of the Limicoline-Gruiform metacentre is found in many of the simpler
Charadriidse such as Numenius, and consists in the gradual shaping of Meckel's tract
into three loops and a supra-duodenal loop. Of the three, one is axial and carries a
large diverticulum on its distal limb. From this Charadri-Gruiform metacentre one line
of progressively increasing apocentricity, consisting chiefly in a lengthening of the axial
loop, is attended by degeneration of the caeca ; it runs through Tringa and Hecurvirostra
to Scolopax. The Gruiform metacentre is another apocentric derivative of the Charadri-

Charadriidae

Fig. 59.

Uicholophidae

Otididas

Charadriidae

Grumae
\

Rhinochetus

Eurypyg.

Columbidae

Opisthocomus

Plerocles

Charadrndcfc

Aramidae

Chiomdab

Gruiform
Metacentre

Charadri-Gruiform
Metacentre

CEdicnemidas

/ Glareolidae
/ / Thinocoridas
///^Parridae

imicoline-

Gniifonn

Metacentre

Palamedea
Archecentre

Evolution of the Intestinal Tract in the Alectoromorphine Legion. (For Helorornis read Ueliornis.)

Gruiform metacentre, and is reached by the first three loops of Meckel's tract, including
the axial loop, being more clearly marked off and lengthening equally. Most of the
Rallidae remain in this condition. The Gruinse are apocentric modifications of it in
which all the loops, except that immediately following the axial loop, increase in length
individually. The Psophiinae and Heliornithidae are practically unmodified from it.
Bhinochetus is modified only in that the first loop of Meckel's tract is represented by
two loops, and that the caeca are reduced. Eurypyga is more modified, its apparent
simplicity being almost certainly secondary. The Dicholophidae and the Otididse are
very apocentric modifications of it, the apocentricity consisting chiefly in the reduction

242

DR. P. CHALMERS MITCHELL ON THE

of the first and third loops of Meckel's tract and the great elongation of the axial and
supra-duodenal loops. Aramus is more difficult to place. It appears to be certainly
more archecentric than other Gruiformes, and is to be derived either from the Charadrii-
form-Gruiforrn metacentre or, even more archecentrically still, from the metacentre
common to the Limieoli and Gruiformes.

CUCULIFORMES.

CTJCTJLI.

CucuLlDyE. — I have examined the intestinal tract of Cacomantis lanceolatus, Carpo-
coccyx radiatus, Centropus (species ?), Crotophaga ani, Gruira piririgua, and Scythrops
novcB-hollandice, and find that it presents the same conformation in all. The duodenum
(fig. 60) is straight and narrow. Meckel's tract is not very definitely produced into

Fig. 60.

sof

Intestinal Tract of Carpococcyx radiatus.

loops, but on its periphery there are two loops anterior to the diverticulum, and, posterior
to it, one very long supra-duodenal loop to which the long caeca are attached. The
rectum is always straight, but differs in length, being extremely short in some Cuckoos.

INTESTINAL TEACT OF BIRDS. 243

. — I have examined several specimens of Corythaix chlorochlamys and
C.persa, and have already figured the intestinal tract (26. fig. 20). The whole gut is
short and wide, in obvious association with frugivorous habit. This circumstance and
the fact that all the specimens I have seen were affected with tuberciilar nodules which
made the examination of the gut unsatisfactory, make it impossible to say anything very
definite regarding these forms. The duodenum is straight and of moderate width.
Meckel's tract appears in two portions, the proximal of which is a short, nearly circular
expanse on which there was no trace of a diverticulum, and the distal a supra-duodenal
loop very closely moulded on the duodenum and supplied by a large bridging vein. The
rectum was short and wide, and I found no trace of caaca.

PSITTACI.

TRICHOGLOSSIDUE : PSITTACID^;. — Of the first family I have examined species of
Trichoglossus and Lorius ; of the second, species of Ara, Cacatua, Conurus, Chrysotis,
JEclectus, Palaornis, Psittacus, and Stringops ; but, as the conformation of the gut does
not differ in any appreciable fashion according to the families and subfamilies, I shall
treat them together. The larger forms have the gut relatively much longer than the
smaller forms, but, under the diversity so caused, the common type is apparent. In all,
the gut is very long and of small calibre ; the walls are thin, and the loops a,re folded
upon each other, and twisted and doubled in a complicated fashion. The masses of
twisted gut are bound together by connective tissue usually loaded with fat, and
"bridging" connections between the blood-vessels in adjoining loops are common. I
have already figured (26. fig. 21) the type as displayed in Ara ararauna. The duodenum
is long, narrow, and usually curved. Meckel's tract is drawn out into very long and
narrow loops, of which the last is always a long supra-duodenal loop with a " bridging "
vein. Of the others, three usually are apparent as in Trichoglossus, and any of these
three may become compound. Thus in Stringops (fig. 61), of the proximal loops
(marked " a " and " b " in the figure), " a " is expanded and shows traces of subdivision ;
" b " is subdivided into three ; " c" appears as two loops, one of which is further
subdivided, and only a comparison with other forms shows that the great length of the
tract has brought about this complexity. I am not confident about the position of
the diverticulum. In Ara there was present what I took to be a small representative
of it at the apex of loop " b" and in one other Parrot I have noted a similarly placed
trace. But in the others, including several other specimens of Ara ararauna, and in
other species of Ara there was no trace. The caaca always are absent, and the straight
rectum varies in length.

The diet of the Cuculidse, consisting of insects, fruit, and flesh, is not of a nature to
have caused much homoplastic modification, and the conformation of the gut is such as
to make it possible to derive it from any fairly archecentric type. Garrod (13) and
others have suggested an affinity between the Cuculidae and the Galli, and, in so far as
the conformation of the gut in Galli is archecentric, there is nothing definitely against
such an origin of the Cuculine gut. On the other hand, there is nothing definitely in
its favour, for such apocentricity as is indicated in the gut of the Galli is different from

244

DE. P. CHALMERS MITCHELL ON THE

that found among the Cuculidse. In these, as we have seen, the tendency is for the
anterior portion of Meckel's tract to be produced into two loops, and this mode of
apocentricity is typical of Coraciiform birds. The reduced gut of the Musophagidae
might have come from any simple source. The Psittaci live largely on hard grains and

Intestinal Tract of Stringops habroplilus.

a, b, c and S.D.F. are the four loops of Meckel's Tract typical of the Psittaci ; a, i, and c being
here subdivided, while S.D.F. is a supra-duodenal loop.

seeds, and this habit, together with the large size of many of the forms, has no doubt
greatly added to the length of the gut and helped to disguise its morphological form.
That conformation is markedly apocentric, as shown by the complicated folding of the
loops, the length of the loops, the individuality of the supra-duodenal loop, and the
absence of caeca. No doubt such a type might have been derived independently from
any of the more arcliecentric forms, from, for instance, some of the simpler Galli, as has
been suggested, but it is equally easy and perhaps more natural to derive the Psittacine
type from that occurring in the simpler Coraciiform birds.

INTESTINAL TEACT OF BIEDS.

245

CORACIIFORMES.

Co RACINE.

CORACIIIXE. — I have examined the intestinal tract of Leptosoma (species ?) and of
Coracias garrula (fig. 62). In both, which are practically identical, the duodenum is a

Fig. 62.

A, I V-V

2s — >3C

S.O./T-

Intestinal Tract of Coracias garrula.

short loop relatively rather wide. Meckel's tract is thrown into two rather wide loops
which are separated by the diverticulum, the latter being conspicuous in Leptosoma,
and only faintly indicated by a trace of a ventral mesentery in Coracias. The distal
loop is a supra-duodenal loop to which the long caeca are attached. The rectum is
relatively short, wide, and straight.

MoMOTiD-E. — I have examined Todus viridis and some other species of the same genus.
The general conformation closely resembles that in the Coraciidas, but the part of Meckel's
tract anterior to the diverticulum is divided into loops and the caeca are shorter.

ALCEDINID.^. — I have examined a number of these. The smaller forms do not differ
in any important respect, save the absence of the caeca, from the conformation in the
Momotidae. In the larger, such as Dacelo (fig. 63), the first of the two loops into which
Meckel's tract is thrown is very long and is wound into a spiral form. In all the
Kingfishers the caeca are absent, but there is a supra-duodenal loop more or less separated
from the general sweep of Meckel's tract, and drained by a " bridging" vein. The caeca
have disappeared completely, and the rectum is very wide and rather long.

SECOND SERIES. — ZOOLOGY, VOL. VIII. 37

246

DR. P. CHALMERS MITCHELL ON THE

Fig. 63.

Intestinal Tract of Daedo gigantea.

Fig. 64.

S.D.F

Intestinal Tract of a species of Merops.

INTESTINAL TRACT OF BIRDS.

247

MKROPIU.E. — I have examined two specimens of Merops, the species not having been
identified. In both (fig. 64) the duodenum was short and rather wide. Meckel's tract
forms an expanded region, the proximal part of which showed trace of production into
t\vo minor loops, while the distal part was a long supra- duodenal loop not well separated
from the general sweep of the Tract. The caeca were not long, but were wide and
contained faecal matter. The rectum was short and wide. In the middle of Meckel's
tract there occurred a distinct diverticulum.

T7.iTPiiu.;. — In Upupa epops (fig. 65) the duodenum, as in many Coraciiform birds,

Fig. fi5.

SD.F.

Intestinal Tract of Upupa epops.
7m, probable position of MeckePs divprticulum.

was a wide loop. Meckel's tract displayed two distinct wide loops on its proximal portion,
while the distal formed a long supra-duodenal loop drained by a bridging vein. There

37*

248

DE. P. CHALMERS MITCHELL ON THE

were no caeca, and the rectum was long, wide, and partly convoluted. la Rhytidoceros
plicatus (26. fig. 23), and other Hornbills resemble it, the duodenum is extremely wide.
Meckel's tract is thrown into three distinct long loops, of which the last is a well-formed
supra-duodenal loop with " bridging " rein. The two anterior loops possibly correspond
to the two loops of Dacelo and other Coracii ; but the second, which is much the longer,
bears a distinct diverticulum near the apex on its distal limb. There are no ca3ca, and
the rectum is rather wide and straight. It is clear that the character of the gut does

O O

not unite the Hoopoes and Hornbills closely.

STEIGES.

(1) STRIGIN^E. — Of these I have examined only Strix flammea. The conformation of
the intestinal tract in this is remarkably archecentric (fig. 66). The duodenum is a large

Fig. 66.

Intestinal Tract of Strix jlammea.

but simple loop. Meckel's tract is fairly symmetrical about the middle mesenteric vein
which runs out towards a diverticulum. It is produced into first one definite loop (marked
"a" in the figure), and then has a circular expanse with a rather narrow neck, uniting
it above to loop "a," and distally to a definite supra-duodenal loop. This expanse is
partly produced into indefinite minor loops. There are two long csera attached to the
supra-duodenal loop, and the rectum is straight and of moderate length.

(2) BTJBONIN.E. — Of these I have examined species of Bubo, Asia, Athene, Syrnium,
Speo-yto, and Gymnoglaux. Of these Bubo maximus, which I have already figured
(26. fig. 22), Asio, and Speotyto do not dilfer in any important respect from Strix; the

INTESTINAL TEACT OF BIRDS.

249

supra-duodenal loop possibly is rather better separated from the general sweep of Meckel's
tract. In the others, except Gymnoglaux, a slightly more apocentric modification has
taken place. In Syrnium aluco, for instance (fig. 67), the duodenum is as in the others.
Loop " a " of Meckel's tract is as in Strix, but the region between this and the diver-
ticulum is much enlarged, so that the diverticulum is no longer at the apex of the circular
expansion. The supra-duodenal loop has increased very much in size, and is well separated

Fig. 67.

Intestinal Tract of Syrnium aluco.

from Meckel's tract, and drained by a series of "bridging" veins, In Bubo maculosus
and B. poensisilie conditions are very similar to that in Syrnium. but the part of Meckel's
tract anterior to the diverticulum appears in two well-marked loops, a conformation
which recalls that in many of the Coraciae. In Gymnoglaux, possibly in association with
its small size, a simplification of the gut has taken place, with the result that there exists
conformation closely resembling that in many of the Rollers and other simple short-
gutted Coraciiform birds. Meckel's tract is thrown into two wide loops separated by
the diverticulum, the posterior loop being a wide supra-duodenal loop with bridging
veins.

250

DR. P. CHALMERS MITCHELL ON THE

CAPRIMTJLGI.

STEATORNITHID^E. — I have examined only a spirit-specimen of Steatornis, and that in a
fragmentary condition after it had already passed through the hands of other anatomists.
It was easy, however, to recognize the general conformation of the gut and its resemblance
to the form in other Coraciiform birds. The duodenum 'was short and rather wide.
Meckel's tract showed two loops anterior to the diverticulum and two, not so well
separated, distad of it, the second being a distinct supra-duodenal loop with the functional
caeca attached to it. The rectum was long and straight.

PODARGID.E and CAPRIMULGIDJE. — I have examined a number of these, several of
which, however, were not identified as to species. In all, the form of the gut was
essentially similar. The duodenum (Nyctidromus albicollis, tig. 68) is short and wide.

Fig. 68.

Intestinal Tract of Nyctidromug albicollis.

Meckel's tract, which is relatively short, appears as a circular expanse, upon the middle
of which the diverticulum was present in most cases, but in some it was absent. The
part of the Tract proximad of the diverticulum showed traces of expansion into the two
loops so common in Coraciiform birds. The part posterior to it had the long caeca
closely applied to it, and these were drained partly by a bridging vein. The rectum was
relatively long and straight. This conformation found in the Caprimulgidse is obviously
remarkably archecentric, and is at once compirable with the primitive type.

GYP SB LI.

CYPSELID^E. — I have examined several species of Swifts, and in all found the confor-
mation similar to that in Cypzelus apus (fig. 69). The duodenum is short and very wide.

INTESTINAL TRACT OF BIRDS.

251

Meckel's tract is short, and forms a circular expanse with the diverticulum nearly at
the central point. The posterior region of the tract, however, shows traces of former

Fig. 69.

Intestinal Tract of Oi/pselits apus.

complexity, in that it is drained partly by a bridging vein, a circumstance to which 1
attach importance as evidence that the apparent simplicity of the gut is pseudocentric.
The caeca are absent, and the rectum is straight and wide.

TROCHILID^E. — I have examined a number of Humming-birds, and found that the
conformation of the gut was in every case practically identical with that in Cypselm.
It is worth while pointing out that the Humming-birds differ clearly in the conformation
of the gut from the Passerine Nectariniidae, although in both the small size has brought
about a remarkable simplicity. The Nectariniidae possess the Passerine nipple-like caeca,
and show clear indications of the spiral twist in Meckel's tract which is a striking
Passer'ne character, and there is no trace of either of these peculiarities in the

Humming-birds.

COLII.

I have already described and figured the intestinal tract of Coitus capemis (26. fig. 21).
It is relatively shorter and wider than in any other bird that I have examined, and this
modification, no doubt due to small size and frugivorous habit, has obliterated practically
completely the underlying morphological form. The gut consists of a very wide
duodenum, a Meckel's tract bearing a small diverticulum, and consisting of a simple
loop similar to the duodenum in size and appearance, and a short, wide rectum. There
are no traces of caeca.

252 DE. P. CHALMERS MITCHELL ON THE

TBOGONES.

In Trogon puella (fig. 70) the duodenum is a large loop expanded towards its apex,
Meckel's tract shows two loops anterior to the position where I infer the yolk-sac to
have been, although there was no diverticulum left as a vestige of it. The distal portion
of the Tract has the long caeca adherent to it, and these are drained by a "bridging"
vein. The rectum is short and straight.

Pic i.
GALBULID^E. — I have not seen any of this family.

CAPITONIB^:. — I have examined Tetragonops Frantzii and Megalcema asiatica. In
these the conformation of the gut is much alike. The duodenum (fig 71) is a short, wide
loop. Meckel's tract exhibits two short loops anterior to the diverticulum, and, distad
of that, forms a distinct supra-duodenal loop with " bridging " vein. There is no trace
of caeca, and the rectum is straight and wide.

IYNGIB^E. — In lynx torquilla the conformation of the gut is practically identical with
that in Megalcema.

RHAMPHASTIDJE. — I have examined Andigena Bailloni, Pteroglossus Wiedi, Rham-
phastos ariel, and R. vitellinus. In all the duodenum is very wide, and Meckel's tract
and the short rectum do not differ from the form found in Megalcema.

PICIDJE. — In Dendrocopus major, Geoinus viridis, and Leuconerpes candidus the con-
formation of the gut is in every important respect similar to that found in Megalcema.

The Coraciiform birds tend generally towards the production of an extremely short,
wide, and simple gut. The small si/e and the frugivorous habit, so common among
them, are agencies that have comhined in producing this pseudocentric simplicity.
Certain forms, such as Nyctidromus, are readily comparable with the archeceutric type,
and present a simple duodenum, a Meckel's tract which is a circular expanse divided by
the diverticulum into an anterior portion and a distal portion to which the L ng caBca
are attached, and finally a straight, rather wide rectum. The changes from such a
condition depend, first, on a widening of the duodenum ; second, on a tendency for the
portion of Meckel's tract proximad of the diverticulum to appear in two loops; and,
third, for a supra-duodenal loop to become distinct in the distal portion of Meckel's
tract. From this condition, various more apocentric modifications of the conformation
are produced, chiefly by shortening and simplification of the tract and by reduction
of the caeca.

PASSEBIFORMES.

I have examined a very large number of P^sscres, belonging to the family Eurylaemida?
of the Subclair.atores ; to the families Tyrannidse, Pittidae, Pteroptochidae, Dendro-
colaptidee, Cotingidse, For mica riidse, and Pipridae of the Clamatores ; of the Suboscines I
have examined Menura, and of the Oscines members of the families Alaudidse, Time-
liidre, Pycnonotidae, Muscicapidse, Turdidse, Troglodytidae, HirundinidaB, Campephagidae,

INTESTINAL TRACT OP BIRDS.

Fig. 70.

253

Intestinal Tract of Trogon puella.

Fig. 71.

Intestinal Tract of Megalcema asiatica.
SECOND SERIES. — ZOOLOGY, VOL. VIII.

38

f

254 THE INTESTINAL TEACT OP BIEDS.

Dicruridae, Ampelidae, Artamidfe, Laniidse, Vireonidse, Sittidse, Oriolidse. Paradiseidse,
Corvidse, Sturnidse, Meliphagidsp, Zosteropidae, Nectariniidse, Dicaeidse, Coerebidse,
Mniotiltidae, Tcteridse, and Fringillidse. This considerable labour from one point of view
had a negative result, for I failed to find in the characters of the alimentary canal any
indication of the families and groups into which systematists have subdivided the
Passerines. The differences that exist among them traverse the group irrespectively of
the divisions of systematists. These differences depend almost entirely on the size and
habits of the birds. The larger forms have a longer gut absolutely and relatively ; the
fruit-eaters have the gut extremely short and wide ; grain- and seed-eaters have the
gut rather longer. But for these differences the conformation is remarkably similar
throughout the group. The duodenum in all is a simple loop, but usually rather wide.
The rectum is always short, and the caeca, although present in every case that I have
examined, are reduced to short rather nipple-like structures of varying shape, but always
with thick walls and only a very small central cavity. Meckel's tract presents the
greatest divergences. The diverticulum is missing in the vast majority of cases. As
an exception, it is extremely large in Menura, and in a few cases it is present. Relying
partly on my own observations and partly on the extended labours of Gadow (12), I do
not doubt that the diverticulum in Passeres is situated on the apical point of the large
portion of Meckel's tract extending from the duodenum to the beginning of the supra-
duodenal loop. In birds with a long intestinal tract, as for instance in Corvus capellanus
(fig. 72), this portion of Meckel's tract is rolled up into a tight spiral which in the figure
is represented as partly drawn out. In birds with a shorter tract, as for instance in
Cyclorhis albiventris (fig. 73), and this may be taken as typical of the vast majority of
Passeres, the spiral is represented by two distinct loops, which, in the unfolded
condition, are applied very closely to one another, and the folded pair of loops have a
slight spiral twist. In all Passeres there is a supra-duodenal loop very distinctly
separated from the rest of Meckel's tract, modelled in the most exact fashion on the
duodemim, and drained by one or more bridging veins from the duodenal vein. The
conformation of the gut in the majority of Passeres, although from its short form it falls
into a very simple pattern, appears in reality to be a highly specialized type, the apparent
simplicity being pseudocentric. In many birds with short guts the intestinal tract is
folded in an irregular and almost capricious manner, and the separate portions bear no
exact relation to one another. In the Passeres the first two loops of Meckel's tract are
most closely applied and extremely similar in calibre and shape, and their arrangement
suggests most strongly that they have been derived by reduction from the proximal
portion of what was originally a long spiral. It would appear to be certain that the
small Passeres have descended from larger birds. In the same fashion, the supra-
duodenal loop is so exactly modelled on the duodenum to which it is applied, as to
suggest specialization rather than simplicity. In Passeres generally, the blood-vessels,
instead of passing through the centre of the mesenteric expanse, run extremely closely to
the intestinal coils, and contribute to the general impression given by the conformation
of the gut as a highly specialized type.

Intestinal Tract of Corvus capellanus ; type of long-gutted Passerine.

.<•, bridging factor of duodenal vein from S.D.F. the supra-duodenal loop ; m, position of Meckel's diverticulum
at the apex of the spiral ; «,.«., d.g., v.n.r/., ganglia of the antonomic nerve-chain.

Fig. 73.

-A-XT

S.D.F.

Intestinal Tract of Cyclorhis albiventris ; type of short-gutted Passerine.

d, apex of duodenum, which in the unfolded condition is applied to p, the apex of the supra-duodenal loop ;
m, position of MeckePs diverticulum, corresponding to apex of spiral in fig. 72.

38*

256

DE. P. CHALMEKS MITCHELL ON THE

SUMMARY or THE CORACIOMOKPHINE LEGION. (Plate 23.)

Gadow (12) unites the Cuculiformes, Coraciiforrnes, and Passeriformes into the
Coraciomorphae, the second legion of his second Brigade. The simplest and apparently
most archecentric type of intestinal tract to he found among these birds occurs in the
Caprimulgi. This, which I call the Coraciomorphine metacentre, is a simple derivative
of the archecentric type, and is changed from that practically only in the shortness and
straightness of the rectum, although the duodenum tends to he rather wider relatively.
From this metacentre the Passerine gut is a very apocentric derivative, the changes

Kg. 74.

Passeres

Psitlaci Alee

Upupidas

Meropidae

inidae

Cuculidee

Coraciforra
Cuculiform
Metacentre

Caprimulgi

Coracimorphme
Metacentre

Palamedea
Archecentre

Evolution of the Intestinal Tract of the Coraciomorphine Legion.
(For Coracimorphine read Coraciomorphine.)

being the specialization of Meckel's tract into a proximal portion which bears the
diverticulum, and which is wound into a long or short spiral, and a distal portion
which is an extremely specialized supra-duodenal loop. The caeca have also been
reduced in size. The conformation of the Cypseli and Colii may also be an apocentric
derivative of the Caprimulgid form, the apocentricity in both consisting of an immense
reduction in the length of the whole gut, with degeneration of the cseca so that no
vestige of them is left, and with obliteration of the loops in Meckel's tract ; there may
or may not be a supra-duodenal loop. The Coraciidoe present a less apocentric moditi-

INTESTINAL TEACT OF BIEDS. 257

cation. Meckel's tract appears in two loops separated by the diverticulum, the posterior
loop having attached to it the long caeca and so forming an incipient supra-duodenal loop.
Underlying the conformations in the other types there is a central pattern which I call
the Coraciform-Cuculiform metacentre, and in which the duodenum is frequently but
not invariably wide ; Meckel's tract is produced into two loops anterior to the diverticulum,
and a single, rather wide supra-duodenal loop to which the long caeca are attached. The
Cuculidtu retain this metacentral form, only slightly altered by increase of length, due
no doubt to the relatively large size of these birds. It is possible to derive the Psittacine
type as a very apocentric modification of this metacentre, the two anterior loops having
become enormously long, and often complicated in themselves, the posterior portion of
Meckel's tract being produced also into at least two complex loops, the distal of which is
a well-specialized supra-duodenal loop, and the caeca having completely disappeared.
However, as the position of the diverticulum is not certain, this derivation of the
Psittacine type can be regarded only as provisional. The Striges present a much less
apocentric modification of the metacentre. The two anterior loops of Meckel's tract are
present, but the second is not so well separated from the general sweep of the tract, and
appears to be part of a circular expansion of the whole tract. But in some of the species
of Bubo the metacentric position is reproduced exactly, and it is easy to regard the
conformations in Strix, Syrnium, and so forth as very slight alterations of the common
form. The Meropidae, the Momotidae, and the Trogones all retain the metacentric
position with extremely little alteration. The Upupidae, the Alcedinidae, and the Pici
are all apocentric modifications of it, the chief difference being the disappearance of
the caeca and the elaboration of a distinct supra-duodenal loop.

SUMMARY OF THE SYSTEMATIC DESCRIPTION.

The various conformations of the Intestinal Tract in birds may all be referred to an
archecentric form, and this form is well displayed in Palamedea. The Struthious birds
(with which the Tinamidae are not included) adhere closely to this archecentric typo.
The Turnices and the Galli adhere to it almost as closely, but among the Galli an
apocentricity, consisting in the expansion of the distal portion of Meckel's tract, begins
to appear. The Tinamidae, the Opisthocomidae, and the Pteroclo-Columbse present
apocentric conformations which are not easy to derive from those in any other groups,
although there are certain suggestions of affinity with the type displayed by Gruiform
birds such as Otis. For the present, however, it is preferable to regard the forms in
these birds as separate, although allied derivatives of the central type. The Passeres
again do not show any clear affinity with the types in other birds. The conformation
displayed must be regarded as having come separately from an archecentric type,
perhaps from that common to the other Coraciomorphine birds, but which has
progressed very far apocentrically along a radius of its own. The conformations in all
other birds may be referred without difficulty to three main metacentres, which are
all simple, but different modifications of the archecentric type. The metacentre of all
the birds in the Pelargo-Colymbomorphine Brigade is a conformation in which the

258

DE. P. CHALMEKS MITCHELL ON THE

symmetry of Meckel's tract about the median mesenteric vein and the Meckel's
diverticulum is retained, but in which Meckel's tract tends to be produced into a
number of long and narrow loops, the most distal of which is a supra-duodenal loop
to which the long ceeca are attached. The metacentre of the Limicoline-Gruiforrn
assemblages (which contains the Alectoromorphine Legion without the Tinamiformes,
Galliformes, and Pteroclo-Columbae) is a derivative of the archecentre in which Meckel's
tract is no longer symmetrical about the middle mesenteric vein and the diverticulum,
but in which the region anterior to the diverticulum is produced beyond it so as to form
an axial loop, on the distal limb of which the diverticulum lies and which may be greatly

Fig. 75.

Pas seres

Coracimorphine
Metacentre

Pelargo-

Colymbimorphme

Metacentre

Limicoli-

Gruiform

Metacentre

Pteroclo- Columbas
Opisthocomi

Tinaim

Turnices

Archecentre
Palamedea

Ratitae

Galli

Evolution of the Intestinal Tract in Aves.
(For Coracimorphine and Colymbimorphine read Coraciomorphine and Colymbomorphine.)

prolonged as in Scolopax. The members of the Coraciomorphine Legion, except the
Passeres and a few pseudocentric types, may be derived from a metacentre in which
the duodenum tends to widen, and in which the part of Meckel's tract anterior to the
diverticulum is produced into two loops. In Plates 21, 22, and 23, and in figures 35, 59,
74, and 75, the evolution and affinities of the conformations of the intestinal tract are
represented, and by no means necessarily the pedigrees and affinities of the birds
containing these conformations.

INTESTINAL PORTION OF THE ATJTONOMIC NERVOUS SYSTEM.

It was only at a late stage of this investigation that my attention was directed to the
peculiar features in the Sympathetic, or, to use the modern term, the Autonomic Nervous
System of Birds. In certain birds (for instance the Megapodidae and Cracidae) the

INTESTINAL TEACT OF BIEDS. 259

so-called intestinal nerve is unusually large and visible, and it was in members of these
families that I first noticed it. In the majority of cases, however, it is not readily seen ;
moreover, much of my material consisted of rather badly preserved spirit-specimens, or
of birds that had died of disease in which tubercular and fatty degenerations of the
mesentery were a conspicuous feature, and I am able only to give a few almost casual
notes, which, however, may serve the purpose of redirecting the attention of anatomists
to a very peculiar and interesting set of structures. A good deal is known about the
paired ganglionated mesenteric chain (G-renzstrang of the German writers), thanks to
the observations of many writers, of whom the chief are Wiedersheim, Gadow, Gaskell,
and Marage. In the cervical region [Wiedersheim (36. p. 350), Gadow (12. p. 394),
and Marage (23, passim)'] it divides into a deep and a more superficial portion, and irregular
traces of this division persist in different birds in the posterior parts of the body. In all
cases it appears to have more autonomy ; that is to say, to be in less intimate connection
with the metameric spinal nerves than in most other vertebrate forms. In the lumbar
region there arises from this a very complex plexus with large ganglia on the edge of the
stomach, on the ovary, on the supra-renal capsule, and further back near the rectum. It
is from these ganglia that the so-called intestinal nerve of birds arises. Concerning this
peculiar nerve the literature is very scanty. Remak (32, 33) appears to have called
attention to it first, but most later writers pass it over almost completely Thus Gadow,
Wiedersheim, Furbringer, and Beddard, in their ornithological and anatomical treatises,
pass it by. Oppel, in his great work on the anatomical tract (30), is content with
discussion of the plexuses actually in the wall of the gut. Gegenbaur (14), in the most
recent edition of his ' Text-book,' mentions the existence of such a nerve in reptiles, and
states that it is best developed in birds ("Am meisten sind diese Nerven bei Vogeln
entwickelt. Ein den Mitteldarm begleitender Nerveustamm geht am Enddarme in
mehrere ansehnliche Ganglien "), and adds that it is undeveloped in mammals. Marage
(23) has given the best account of it, and states that it differs in its arrangement in
different birds ; but his figures are rather difficult to follow, and he is incorrect in stating
that it does not occur in rapacious birds, and he seems to have overlooked it in the
Ratites. As I have already stated, I do not pretend to have made anything that
approaches to a complete study of it, and the figures that I have been able to give must
be taken only as rough anatomical notes. Special study by special methods on material
in good condition is required *. I can say, however, that in every case since I was
aware of its existence, where the material was in sufficiently good condition, I found it
present, and I do not doubt that this presence is invariable. It arises usually from two
or three main nerves, which leave the ganglia corresponding to the solar plexus and the
ganglia over the ovary and supra-renal capsule, and enter the mesenteric expanse which
is the support of Meckel's tract. These, or some of these, are represented in figures 1
and 45, and I have worked them out in several unfigured cases. In fig. 72 there is a

* Since writing this memoir, my attention has been directed to a beautiful memoir by The'bault (34), which
contains, inter alia, an elaborate study of the modes of origin of the intestinal nerve in various types of birds. —
P. C. M.

260 DR. P. CHALMERS MITCHELL ON THE

large ganglion at entrance of these to Meckel's tract, and I could make out only two
nerves forming the origin of this ganglion. One of the three nerves frequently runs
independently to the duodenum, as in Otis (fig. 45) and in Andigena Bailloni ; but no
doubt there may have been a cross connection between this and the main intestinal
nerve, as certainly is the case in the Pigeon. In the duodenal-fold loop usually at least
one distinct and large ganglion is present (fig. 45, v.nx. ; fig. 72, d.g.}. The main
intestinal nerve which supplies Meckel's tract starts in common with the duodenal
nerve as in Palamedea (fig. 1) and the Passeres (fig. 72). It courses round the mesen-
teric expanse of the tract, giving off a series of minute branches to the gut, and where the
tract passes into the rectum it frequently receives one of the entering nerves, or is in
connection with the ganglion at the entrance to the mesentery of Meckel's tract, and
then turns down parallel to the rectum, and comes again into connection with the
posterior portion of the Grenzstrang. On its course this main intestinal nerve may
follow Meckel's tract more or less closely. Thus in Palamedea (fig. 1), Pandion (fig. 34),
Talegallus (fig. 38), it forms a loop corresponding to, but not absolutely identical with,
the course of the tract. In Falco melanogenys (fig. 32) it follows the general course of the
tract much more closely, having a loop following the supra-duodenal loop. In Crax
Daubentoni (fig. 39) it has a curious elongated and narrow distal portion, which corre-
sponds exactly with the typical apocentricity of these birds, in which the tract itself is
enlarged on its distal portion. This intestinal nerve in many cases — Struthious birds,
Palamedea (fig. 1), Talegallus (fig. 38), Crax (fig. 39), other Galli, Falco (fig. 82),
Pandion (fig. 34) — presents a very large number of small ganglia evenly distributed along
its course. In other birds, e. g., Otis (fig. 45), Coluniba, Andigena, and Corwus (fig. 72),
there are a smaller number of much larger ganglia. As to the systematic value of these
differences, it is impossible to say anything definite on so small a range of information ;
but the subject is very promising. It appears as if the chain with many ganglia were
more primitive than the nerve with a limited number of large ganglia ; and it is certainly
the case that such birds as Otis, Coluniba, the Toucan, and the Passeres, where the
number of ganglia is very small, are more specialized forms than those with multi-
ganglionated chains. It has to be remembered, however, that although the nervous
systems of many invertebrates with a small number of large ganglia appear to have been
produced by the concentration of multi-ganglionated chains, we have no right to extend
such a principle to other forms. The origin of this intestinal nerve in birds requires to
be worked out, and in this connection the observations of Andersson (i), who found that
in the Urodele Amphibia the main sympathetic chain was subdivided into a Grenzstrang
and a Collateralstrang, are worthy of attention.

It is to be noticed that this intestinal nervous system lies, as the blood-vessels lie,
between the two layers of the mesentery, and therefore outside the crelom. In larger
forms, such as the Emu, it is often comparatively easy to strip off one layer with its
load of fat from the mesenterial expanse supporting Meckel's tract, upon which the
ganglionated nerve-chain come plainly into view. However, just as it occasionally
happens that some of the blood-vessels pierce the mesentery and form •' bridging " vessels
traversing the portion of coelom between two loops which happen to lie in contact, so in

INTESTINAL TRACT OF BIKDS. 2(51

certain cases nerves may " bridge," or rather bore through, the mesentery and pass across
a narrow portion of coelomic space. The most obvious case of this occurrence is where?
as in Otis, nerves leave a ganglion in the duodenal loop and pass to the caeca or the
supra-duodenal loop. I am practically certain that this happens in many of the small
Passerines, where the supra-duodenal loop is a structure of considerable importance, but
I am not yet prepared to demonstrate this.

MORPHOLOGY OF THE INTESTINAL TRACT.

The chief writers on this subject Avithin comparatively recent times have been Toldt
(35), who deals chiefly with the tract in Man; Klaatsch (18), who in two extremely
important memoirs dealt with the relation of the tract and its ancillary viscera to the
mesentery; andMathes(24), who followed closely the work of Klaatsch, but dealt chiefly
with the development of the mesenteries in the Amphibia. None of these writers has
paid special attention to the conditions that exist among .Birds, but their work has been
of great assistance to me in interpreting ;md coordinating my own investigations. It is
plain that in all the higher vertebrates the intt-stinal tract is thrown into three main
portions which are homologous throughout the series. Of these, the first is the
Duodenum, which in birds is always a closed loop lying ventral to the rest of the tract.
Jt arises extremely early in ontogeny, and while in the majority of cases it remains
simple, it may develop many minor complexities, sometimes simply becoming wider,
sometimes being thrown into numerous minor folds, and sometimes being twisted into
more or less regular spirals, the spiral duodenum being in some cases (Storks) wound
with the spirally twisted proximal loop of Meckel's tract. Concerning the relations of
the duodenum to the supra-duodenal loop I shall presently have more to say. The
distal extremity of the duodenum, however the course of that may have been complex,
always returns to the dorsal edge of the mesentery very close to the starting point of the
anterior limb, and there passes into the second portion of the gut. This portion, which I
name Meckel's Tract, extends from the duodenum to the insertion of the caeca. The
first important point about this large region of the gut is that it represents an outgrowth
of only a very small section of the primitive gut. Its proximal extremity approaches its
distal extremity so closely in the line of the dorsal attachment of the mesentery, that in
the majority of cases it would be possible to remove the whole of Meckel's tract and
surure the cut edge of the duodenum to the cut proximal edge of the rectum, and
a. most without dislocation reconstruct a primitive straight intestinal canal. In actual
development Meckel's tract, in all the vertebrates in which it is developed, arises as a
simple narrow loop in the line of the principal mesenteric artery. Toldt's figures, and
others given by Kollman (19), show this beautifully in the case of human embryos, and
general comparative anatomy from the Frog to Man makes the morphological nature of
Meckel's tract extremely plain. There can be no doubt that this is the most recent
phylogenetic development of the Vertebrate gut, and that it corresponds to not more
than two, or possibly three, of the primitive somites of the body. "When the development
and comparative anatomy of the intestinal nervous chain in Birds has been worked out,

SECOND SERIES. — ZOOLOGY, VOL. VIII. 39

202 DE. P. CHALMERS MITCHELL ON THE

it is to be expected that the origin of that system from not more than three entering
nerve-branches will be given an importance which as yet can only be suspected. The
nature of Meckel's tract as a recent outgrowth of a portion of the gut corresponding
only to a very limited number of segments has an important bearing on medical work,
as it is being found that there is an organic sympathy through the nervous system
between certain superficial areas of the skin and certain visceral organs or portions of the
organs ; the sympathy depending on a common relation to the primitive segmentation of
the body, and being of practical value in the diagnosis of affections of internal organs.
In those creatures in which there is a yolk-sac or umbilical cord, the outgrowth of
Meckel's tract from the primitive straight gut is opposite the vessels running to these
structures ; and it seems tempting to regard the origination of Meckel's tract as being
dependent on this mode of embryonic nutrition. However, as Meckel's tract is equallv
definite and equally rnetamerically limited in the Frog, we cannot lay much stress on
this possible mode of origin.

The large intestine, from the caeca to the cloaca or anus, is the third distinct portion of
the intestinal tract. It always lies in the primitive position of the straight gut, dorsad of
all the other portions, and phylogenetically it is the oldest portion and corresponds to the
greatest number of somites. It appears to be homologous throughout the vertebrate
series ; but this homology depends on the identification of the paired cadca of Birds with
the unpaired cascum of Mammals and Lizards — a homology not apparent when a bird
with a short gut is taken, but much more convincing when the comparison is made
between a form such as Palamedea (fig. 1) or a Struthious bird and a Mammalian or
Lacertilian intestinal tract. In the more primitive types the large intestine is very long,
and may show traces of division into colon and rectum ; in the higher forms, partly in
correlation with the greater development of the thoracic portion of the viscera, the
duodenum and whole intestinal tract has shifted far distally, with the result that the
large intestine is extremely small, and is here referred to simply as rectum (see footnotes,
pp. 176 and 271).

MECKEL'S DIVERTICULUM.

Examination of the embryo of any bird makes it plain that this structure is the
vestige of the yolk-sac, and its retention in adult birds has been described by a number
of anatomists. It has been termed by most writers who understood its nature the
Diverticulum ccecum vifelli; but as it is obviously homologous with the caBcuin described
by Meek el as an abnormality in Man, and as in human anatomy it bears the well-known
name of Meckel's diverticulum, I have preferred to give it that name in birds. I have
not found it present in any adult reptile, and I can find no record of its occurrence
there. Nor have I found it in any mammal other than Man. In human anatomy its
frequency of occurrence has received recent attention, and Birmingham (4) states that it
is present in about 2'2 per cent., varying from half an inch to five inches in length. In
birds it is present much more frequently ; indeed, its presence throughout life is a
character of very many groups. Gado\v (12), summing up his own very numerous
observations and those of earlier writers, states that it is retained throughout life by

INTESTINAL TEACT OF BIRDS. 263

Swimming-birds and most Waders, but that it disappears very early in Birds of Prey,
Parrots, Woodpeckers, and Singing-birds. He regards it as a functionless rudiment.
Lonnberg and Jagerskiold (21) examined a large number of birds, chiefly Sea-birds and
Waders, for it, not contenting themselves with single individuals, and found it absent in
most Gulls and Terns, present in Waders, Ducks, Herons, and some others. As at an
early stage in my investigations I found it an important point of morphological orien-
tation, I searched for it in each of the very large number of birds upon which this
communication is based, and found its presence much more frequent than has been stated.
An interesting feature in connection with it is, that in a large number of cases it is sup-
ported by a vestige of the primitive ventral mesentery. Usually this appears only as a small
fold of tissue tying down the diverticulum to the ventral edge of the gut, but sometimes
a strong band runs from this towards the liver, the latter of course being developed in
the primitive ventral mesentery. Occasionally when the diverticulum itself was absent,
its place was indicated by the presence of a mesenterial vestige, as in many Pigeons.

I have found the Diverticulum present and large in all the Ratitse ; of the Colytnbiform.es
absent only in Podiceps ; present in the Sphenisciformes ; present in the Procellariiformes ;
of the Ciconiiformes, present and large in all the Steganopodes but small in Plotus, and
present and large in the Ardeidse, Scopidae, Ciconise, and Phoeuicopteri, in many of the
Ciconiiformes being very large ; present and large in all the Anseriformes ; present in all
the Falconiformes (except a Buteo), and very large in the Falconidte, but usually small
in the others ; present and large in the Tinamiformes ; present but usually very small in
the Galliformes (except Turnix) ; present and very large in all the Gruiformes, often
extremely large, but, as a solitary exception, absent in Heliornis ; present and very large
in all the Limicoke; among Lari small and often absent in the Laridse, small, but
usually present, in the Alcidse ; present in the Pterocletidae, but usually absent and
always extremely small in the Columbidae, a feature in which they stand in marked
contrast with the Charadriiformes generally ; of the Cuculiformes, always present
although small in the Cuculidse ; absent in the Musophagidae, and probably always
absent in the Psittaci ; of the Coraciiformes, always small, but present rather more often
than not in the CoruciaB, present and small in most of the Striges, very small but present
in most Caprimulgi ; always very small and absent more often than present in the
Cypseli, Colii, Trogones, Pici ; of the Passeriformes, absent in the vast majority of cases,
when present extremely small except as a solitary case in Menura, where it was very
large. In making this review I have excluded those specimens which were marked in
my notes as obviously chicks or quite young birds, and the list may be taken as repre-
senting with fair exactness the incidence of the diverticulum among the Avian groups.

In all the c;ises where I have mentioned, in the paragraph above, that Meckel's diverti-
culum was small, I think that it was a vestige in the true sense, that is to say a functionless
rudiment of an embryonic structure. Sometimes the lumen remained, and contained a
few fragments of material resembling yolk, and doubtless remains of the yolk. Rather
more frequently the " small " divei ticula had no lumen, and were mere nodular
excrescences on the wall of the Tract. Among the cases which I have noted as large there
are certainly some (Ratites, Palamedea, etc.) in the same category. The curious feature

39*

2(54 DR. P. CHALMERS MITCHELL ON THE

therefore exists that a functionless vestige is retained universally in some groups and
not in other groups — a circumstance to which the attention of those naturalists may be
directed who would see a purpose or "selection-value" in every systematic character.

Meckel's diverticulum presents another condition of great interest. Lonnberg and
Jagerskiold (21) drew attention to the fact that in certain cases where the diverticulum
is large, it has a patent lumen and a thick wall, and is slightly constricted from the gut
(21, cited in Oppel 30, p. 559). These authors, while they suggest that the vestigial
organ has been transformed into a gland, do not go further in their microscopical
investigation than to point out that the mucous membrane of the diverticulum in these
cases is thickened by a series of folds. I have examined the microscopical structure in
several Anatiformes, Gruiformes, and Charadriiformes, and find that glands occur
frequently in the foldings of the mucous membrane, and that the diverticulum in such
cases corresponds, with its folded wall containing glands and lymphoid nodules, very
closely to the structure of the caeca in Passerines and Columbae. I am inclined to think,
therefore, that in many cases, particularly in the groups that I have mentioned (and
possibly in the Falconinae), Meckel's diverticulum has acquired a new function. It is at
the least suggestive that where the diverticulum has become glandular, the paired caeca
are either rudimentary and functionless, or they are very large, thin-walled, and full of
faecal matter. "Where, on the other hand, the paired caeca are chiefly glandular, the
diverticulum is either a functionless vestige or has disappeared. This, hoAvever, is not a
complete account of the relations of the two organs ; for, in the first place, it is based only
on a relatively small number of observations of microscopical structure, and in the second
place there are instances, such as the Psittaci, where both diverticulum and paired cseca
are absent in the adult.

THE SUBSIDIARY LOOPS OF MECKEL'S TRACT.

In the section dealing with systematic description, I have already said all or nearly all
that 1 have to say regarding the minor loops into which the Tract so frequently is
produced. I have tried to show that these display patterns which persist through
systematic groups, the persistency referring to their position with regard to the diverti-
culum and to their number. The arrangements of these minor loops, in fact, are
instances of what I term uniradial apocentricities. When Meckel's tr.Mct is elongating,
in a large bird or in a bird the hahits of which demand a great length of gut, the
elongation does not take place at random, but in special regions and in special modes.
The combination of position and complexity is of a kind not likely to be repeated
independently, but to have common origin, and so to prove of systematic value. The
explanation of how these different and complex combinations came into existence I have
not attempted. Before that could be done, there is necessary the great labour of
following out in every stage of individual development the relations of the growing folds
of the gut to the blood-vessels, regions of the coelom, liver, air-sacs, sternum, stomach,
and so forth. The beautiful work of Klaatsch and of Mathes, elaborate and prolonged as
it was, does little more than to open up the lines of such enquiries. Until something
is known in each individual case of the nature of the " growth-forces " in contiguous

INTESTINAL TRACT OF BIRDS. 265

organs, of their compromises and co-ordinations, in fact of their places with regard to
one another and to the whole corporeal republic in every stage of the growing embryo,
nothing more can be said but that such complex uniradial apocentricities, if not in them-
selves possessed of " selection-value," may stand in. correlation with structures that have
such value.

THE SUPRA-DUODENAL Loop.

In its natural condition the duodenum lies folded vent rally uuder the other portions
of the gut, and comes in very close relation with the distal portion of Meckel's tract.
Cuvier drew attention to the fact that in birds as in mammals the duodenum comes into
intimate relation with a posterior portion of the gut, a relation so intimate that the
mesenterial folds suspending the two portions may fuse after the fashion explained bv
Klaatsch. This portion of the gut which comes into relation with the caeca I have
called the supra-duodenal loop, preferring not to call it " colon " as was done by Cuvier,
since that term is applied in Mammalian anatomy to a portion of the gut posterior to
the caeca, and therefore belonging to what I have been terming the rectum. Within
the group of birds various stages in the evolution of this curious inter-relation
between the proximal and distal portions of the gut are displayed. Thus in Palamedca
(fig. 1) there is no supra-duodenal loop, and the gut may be unfolded witiiout any
difficulty or cutting of blood-vessels. The same conditions obtain in a number of the
archecentric types, and in the systematic portion I have referred to these. Even in
Palamedea, however, the earliest stage in the formation of the connection is apparent.
A short recurrent factor of the duodenal vein runs in the mesentery at the dorsal portion
of Meckel's tract across from the terminal portion of that tract and assists in draining
the cseca. As the caeca increase in size this vessel becomes larger and of more
importance, and, in many cases which I have referred to in the systematic portion, the
recurrent vein along with a second and sometimes a third accessory recurrent vein run
to nearer the apex of the duodenum, having traversed the mesentery as " bridging " veins,
and arisen from the long caeca and the posterior portion of Meckel's tract. In such a
mode, an intimate relation is established between the duodenum and the distal portion of
Meckel's tract, and, as in Otis, this relation may involve not only the veins but the
nerves. The portion of the Tract to which the caeca are adherent is not distinctly
marked off from the more proximal portion of the Tract in very many of the less
apocentric types. But, as I have shown in the systematic portion, pari passu with the
establishment of the " bridging" veins, the distal portion of the Tract becomes a distinct
loop clearly marked off from the general sweep of the Tract. Finally, in those birds
where the caeca have degenerated either completely or have shortened to glandular
nipples, the supra-duodenal loop is retained with its separateness from the rest of the
Tract and with its " bridging " veins. In such a form the supra-duodenal loop generally
becomes very accurately moulded to the contour of the duodenum, and its presence and
completeness are important reasons for seeing in the apparent simplicity of the gut in
many of the higher forms, such as the Passeres, a pseudocentricity — a condition apparently
simple, but still retaining evidence of past complexity. I do not think it can be doubted

206 DR. P. CHALMERS MITCHELL ON THE

that wherever in short-gutted birds without large caeca the supra-duodenal loop is present
in its elaborate form, these birds at one time had a longer gut, and almost certainly
possessed longer caeca. Where a gut is long, and where its irregularly folded loops lie
closely together in a narrow space, the establishment of an intimate connection between
any of the crowded loops would call for little remark. Where, as in the Passeres and
many other short-gutted forms, there appears to be abundant space, and yet the
duodenum and the supra-duodenal loop are accurately moulded, the one on the other,
and in intimate vascular, nervous, and mesenterial connection, we must seek for the
origin of the complexity in the past history.

There can be no doubt, I think, that the presence of a specialized supra-duodenal
loop marks a high degree of apoceutricity in the intestinal tract possessing it, but it is
equally plain that this apocentricity is multiradial and no guide to affinity. The
presence of caeca of at least moderate length is a fundamental or archecentric character
of birds ; and, if the supra-duodenal loop has arisen in the mode I have indicated, it is
clear that it may have arisen repeatedly, and in my systematic description I showed that
actually it does appear, repeatedly and apparently independently, in the different groups.
The probably multiradial nature of the structure is also supported by the occurrence
of similar formations among Mammals, these formations not even being on exactly
homologous parts of the gut. The mammalian structures with which the Supra-
duodenal Loop may be compared are the loop which in Man has the transverse colon as
its apical portion, and the sigmoid flexure, which in embryonic Mau reaches much
further towards the duodenum. Naturally, I do not propose to enter at present into the
various modifications presented by these structures ; it is enough to say that the
connections in Mammals between the duodenum and posterior regions of the gut are
frequently present, but less frequently than in the case of Birds.

Dealing with its occurrence in Man, Toldt sees in these connections between the
proximal and distal portions of the gut the mere result of apposition. Klaatsch
criticises this view, and rightly points out that, although many of the other loops are in
equally close spatial relations, fusion does not necessarily occur among them. In Birds,
however, as I have shown, secondary fusions and " bridging " veins are not absolutely
confined to the duodenum and supra-duodenal loop. Thus in Psittaci they are frequent
among the numerous long and narrow loops into which Meckel's tract is thrown. In
Anatidae secondary fusions and " bridging " veins are of frequent occurrence in the case
of different portions of the same loop, notably in the axial loop, and in Spatula in the
long loop distal to Meckel's diverticulum (fig. 24). Finally, in the Cicouiae bridging
veins and secondary fusion of the mesentery occur between the duodenum and the
proximal minor loop of Meckel's tract. These occurrences, however, are less important
and much less frequent than the supra-duodenal connections. Klaatsch accounts
for these in the case of Reptiles and Birds by the mode in which the coelomic divisions
are broken up by the intruding blood-vessels and viscera. I think, however, that, in
birds, the formation may serve a useful purpose ; the supra-duodenal loop is maintained
and made even more elaborate after the degeneration of the caeca, although it arose in
relation to the cseca ; moreover, it is specially perfect in cases where there is no reason to

INTESTINAL TEACT OF BIRDS. 267

suppose that want of space played a part in its ontogenetic appearance. Plainly, when
thepyloric valve relaxes and the contents of the stomach are poured into the duodenum,
the shock will be transferred to the supra-duodenal loop which lies closely applied to
the duodenum and sometimes in special nerve-connection with it. And thus discharge
of the contents of the posterior region of the gut into the rectum may he set about
without the necessity of peristaltic waves traversing the whole length of Meckel's tract.

THE COLIC C^ECA.

Gadow (12), Furbringer (9), Beddard (2), and Oppel (30), in their respective treatises
have devoted so much attention to the voluminous literature concerning the colic caeca
of birds that I need not refer to older writers. The archecentric condition of these
organs in birds, a condition which is probably an heritage from lieptilian ancestors, is the
existence of a pair of caeca growing from the point where the distal end of Meckel's
tract passes into the rectum. Such primitive caeca proximally are applied more or less
closely to the posterior portion of Meckel's tract. They are of moderate length ; their
walls are not specially thickened, their lumens are widely open to the gut, and their
contents consist of food-material in a state more akin to that in the rectum than to the
state in Meckel's tract. When, as happens frequently, there is a difference in colour
apparent through the intestinal wall and marking the different stages of metabolism, the
colour of the caeca approximates to that of the rectum. These primitive caeca probably
had a digestive function of some sort, for the presence in their walls of glands, of
absorbing veins, and occasionally of villi show that they were not mere reservoirs of
faecal matter. From the primitive condition various apocentric modifications have
arisen. The caeca may increase very greatly in size, and may develop spirally arranged
septa protruding more or less into the cavity and deeply marking the exterior, as in
Chauna and some of the Ratites and Gruiformes, or the whole external surface may be
prolonged into a number of papilliform hollow outgrowths, as in Calodromas described
by Beddard (2). These enlarged caeca appear to retain their digestive functions.
Secondly, the caeca may become very much reduced, but in such apocentric reduction I
am convinced that there are two quite different conditions to be noted. In the one case,
the reduction may he nothing more than the degeneration of an organ that has become
f unctionless ; and almost any stage from the archecentric size to complete absence may
exist. In the Columbidae, for instance, the caeca if present are always small and
frequently are thin- walled, irregular (I have noted many cases of individual absence of
one caecum), and sometimes pigmented. In the fruit-eating Pigeons, and indeed in
many other Pigeons apart from the nature of their food, the vestiges have disappeared
completely. Precisely a similar series of events occurs among most Falconiform birds.
The caeca are small in all, and when present appear to be functionless, thin-walled
vestiges, frequently unsymmetrical (here, again, absence on one side has been noted in
different birds by different observers). Sometimes they appear to be absent, but inflation
of the wall of the gut reveals the presence of slight, thin-walled rudiments in their
place. In the Vultures they are sometimes totally absent, and a similar absence is a

2G8 DR. P. CHALMERS MITCHELL OX THE

character of the Cathartidae. However, there is yet another condition possible among
these reduced caaca. Numerous observers have shown that in the caeca of many birds,
whether these be long or short, there occur in different regions masses of lymphoidal
tissue. Such masses may occur at the apex of long caeca as in the Owls, or in scattered
patches as in Ducks and Fowls, or in concentrated swellings as in Otis. Berry (3) has
recently shown that the occurrence of these lymphoid masses, in concentrated regions
forming vermiform appendices, or in scattered masses, is frequent in Vertebrates, and he
suggests that the vermiform appendix of Man is a specialized and not a degenerate
structure. The reduced caeca of many birds (e. g. Passerines) are quite different from
the obviously vestigial caeca that I have mentioned. They are regularly formed, nipple-
like structures, the lumen of which is greatly reduced by closely-set villi and folds. In
these, small glands and lymphoid tissue are abundant ; and in the Sparrow I have noted
the presence of abundant dark granules which are apparently matter in process of
excretion. I distinguish therefore in the conditions of the caeca among birds the
following : —

1. Arcliecentric Character. — Caeca large, thin-\valled, containing food- material on

which some process of digestion is taking place, but which may have in addition
the beginning of secretory or excretory function.

2. Apocentric Characters.

A. Caeca enlarged, partly digestive, and partly secretory or excretory.

B. Caeca functionless, vestigial or absent.

G. Caeca reduced but transformed solely to glandular organs, secretory or
excretory or both .

PALAMEDEA. Caeca archecentric. Chauna apocentric A.

RA.TITJE. Ctecae archecentric or apocentric A.

COLYMBIFORMES. Arcliecentric.

SPHENISCIFORMES. Apocentric B. but according to Gadow archecentric in some.

PROCELLARIIFORMES. Apocentric B, occasionally one only present.

CICONIIFORMES. Arcliecentric in the Pelecanidae, apocentric B in the Phaethontida-,
Sulidap, Fregatidae, and Phalacrocoracidae (in Plotus one may be absent) ;
apocentric B in the Scopidae, Ciconiidae, and Ardeidae, in the latter one only as
the normal but not invariable condition. In the Phcanicopteri urchecentric.

ANSERIFORMES (excluding the Palamedeidae). Apocentric A, usually with large rows
of glands, but in Mergus apocentric B and one occasionally absent.

FALCONIFORMES. Apocentric B, vestigial with frequent irregularities and absent in
Cathartidse.

TINAMIFORMES. Apocentric A, extreme specialization in some.

GALLIFORMES. Apocentric A in Galli and Opisthocomi ; archecentric in Tunuces.

GRUIFORMES. In the Kallidae apocentric A; of the Gruida^ archecentric in the
Araminae, apocentric A in the others. In the Dicholophidae and Otididae
apocentric A (extremely so in Otis); in The Rhinochetidsc archecentric; in the
Eurypygidae apocentric, probably C ; in the Heliornithidau archecentric.

INTESTINAL TRACT OF BIRDS. 269

CHARADRIIFORMES.

LimicoloK. In the Glareolidae archecentric ; in the Chionidae, Thinocoridse, and
Charadriidae apocentric A, but in some of the latter family apocentric C. In
the Parridae, Alcidse, and most of the Laridse apocentric B and sometimes
practically quite absent, but in Stercorarim archocentric.

Pterocletidce. Apocentric A.

Columbida;. In the vast majority apocentric B, occasionally one and very often
both absent, but in Columba livia and Columba cenas traces of glandular
structure and so apparently apocentric C.

CUCDLIFORMES.

Cuculi. Archecentric to apocentric A in the Cuculidse ; in the Musophagidae
apocentric B, and usually, if not always, quite absent. In the Psittaci
apocentric B.
CORACIIFORMES. In the Coraciidae, Momotidae, and Meropidae archecentric ; in the

Alcedinidae and Upupidae apocentric B.
Striges. Apocentric A.
Caprimnlgi. Archecentric.
Trogones. Archecentric.
Cypseli, Colii, and Picl are apocentric B.
PASSERIFORMES. All that I have examined are apocentric C.

Gadow (12. pp. 688-693) has made a very careful study of the relations between the
character of the caeca and the nature of the diet, and I have gained further information
from Finn (8) and from Evans (7), and from the keepers at the Zoological Gardens as to
the diet of birds. There is very far from an exact correlation between the apocen-
tricities and particular diets, but there is enough to make it plain that many of the
modifications are homoplastic ; that is to say, that they are multiradial apocentricities
on which no argument as to affinity can be based.

Archecentric Type. — The diet in these forms is by no means uniform, but there is
a preponderance of feeding on insects and caterpillars, grubs, and so forth ;
in fact, of what may be taken as more typical reptilian diet.

Apocentric Type A. A very large number of birds displaying much enlarged ca3ca
live on vegetable diet in which cellulose bulks largely, and it is to be remembered
that the digestion of cellulose occurs in the large caeca of some mammalia.
Apocentric Type B. Carnivorous diet, piscivorous diet, and especially frugivorous
diet are specially associated with the degeneration of the ca?,ca to functionless
vestiges, or even with the complete disappearance of the caeca.

Apocentric Type C. When it is remembered that the vast majority of the Passeres
display this type, and that yet almost every possible form of diet is found among
them, it is at once clear that diet is no complete explanation of the character of
the caeca. I am more inclined lo think that type C is to be associated with a
high development of the Avian structure, and that it is a peculiar secretory or
excretory organ.

SECOND SERIES. — ZOOLOGY, VOL. Till. 40

270 DE. P. CHALMERS MITCHELL OX THE

Some parallels may make the complexity of the problem of the caeca apparent. The
Passeres display all diets and one type.

The Owls and the Falconiformes have almost identical diet, the larger forms of both
being carnivorous, the smaller forms chiefly insectivorous ; the Owls all exhibit apocen-
tricity A, the Falconiformes apocentricity B.

Fowls and Pigeons (except the fruit-eating Pigeons) have a similar diet ; Fowls all
exhibit apocentricity A ; Pigeons apocentricity B or rarely C.

The Gulls have a similar diet consisting of fish and garbage ; most exhibit apocentricity
B, but Stercorarius is archecentric.

On the other hand, Phcenicopterus is distinguished from other Ciconiiformes by a
nearly purely vegetable diet, and it has the longest caeca in the group. The Mergansers
are distinguished from the other Anatidse by their typically piscivorous diet, and they
alone among the Anatidte have apocentricity B as in most other fish-eaters.

CHARACTERS AND CLASSIFICATION.

In the systematic descriptive part, my task was to treat the characters of the patterns
displayed by different birds as nearly as possible as if the gut were the whole animal, and
the various phylogenetic figures and the three Plates display what I take to be the
relations of the intestinal tracts, and not necessarily the relations of the possessors of
these tracts. I have been taking, in fact, the anatomical structure as the unit, and not
the individual or the species. In a sense, this is a return to the old Hunterian method ;
but its purpose reflects on the new problem of the nature and evolution of varieties and
species of genera and families. Granting that the Plates attached to this paper represent
with approximate accuracy the phylogeny of the intestinal tract in birds, we have yet to
learn the relation of the phylogenetic tree of this structure to the phylogenetic trees of
other structures, and the relation of all these to the phylogenetic trees of these im-
permanent combinations of characters that we call species. Although the coincidence of
such trees is frequently assumed, there is no a priori reason to support such a proposition ;
and there is much recent work on the nature of characters and of their inheritance to
throw doubt on the proposition. The nature of the anatomical structure in any organism
depends in the first place on the nature of certain material transmitted from the parents
in the fertilized ovum (naturally it does not matter to the argument whether the trans-
mitted substance be what we call "matter" or "state of matter" such as mode of
rhythm). Among other writers de Vries (6) has recently brought strong experimental
evidence to show that at least in plants the hereditary material is composed of inde-
pendent units which may be sorted out and recombined in each sexual generation. Of
such independent variables underlying the fully developed anatomical structures of
animals, we know practically nothing as to their number, nature, or modes of sorting
out and recombination as they pass from generation to generation. Next, the anatomical
structures of animals depend on the environment in which the combination of transmitted
units come to maturity as actual adult organs ; that is to say, they depend on the

INTESTINAL TRACT OF BIRDS. 271

various correlations with other organs and with external conditions established during
embryonic, youthful, and adult life. In the case of the alimentary canal we know just
enougii, from the transforming experiments of Hunter and others, to feel certain that
a large part of the final structure is determined by the conditions in which the hereditary
material is grown. The phylogenetic tree of an organ such as the alimentary tract may
be little more than the exhibition of a long series of experiments in growing similar or
identical " germs " in different culture media. And, finally, we know nothing as to how
far these hereditary masses, as they were marshalled into the actual branches of the
actual phylogenetic tree of the organisms that contained them, carried within themselves
historical limitations or determinations towards further development in specialized
directions.

Such questions, however they may seem remote from an actual study in comparative
organogeny, in reality lie at the root of every attempt to use characters in classification :
if by classification there is implied more than the convenient disposal of particular
animals in particular pigeon-holes according to their possession of obvious anatomical
characters. The study of characters by themselves and for themselves must precede
the attempt to use these characters in genealogical classification.

In the study of characters, the first proposition is whether they are primitive or
modified, archecentric or apocentric. So far as the subject-matter of this paper is
concerned, I have tried to show that for Aves the presence of a specialized duodenal
loop, of a Meckel's tract consisting of a nearly circular expanse of mesentery with a
simply convoluted gut suspended at its periphery and symmetrical about a median vein
running towards a vestige of Meckel's diverticulum, of a pair of colic Cfeca with free
lumen of moderate length and with walls partly glandular and partly absorbing, and of
a rectum relatively long, are archecentric characters. When birds possess many or even
all of these characters, we are tempted to say, but we cannot say definitely, that they are
closely akin. The retention by some descendants of a common stock of an ancestral
heritage in the ancestral form does not imply that such members are more closely akin
than other members that have dissipated or improved the common heritage. I have
shown that in all the great accepted groups of birds there are some members with the
archecentric condition.

The second proposition that may have to be made regarding characters is that they
are modified from the archecentric condition, or apocentric,- and in this modification
degrees of apocentricity may be exhibited. I have shown that the lengthening and
twisting of the duodenum, the appearance of asymmetry in Meckel's tract and its produc-
tion into variously-formed and variously-placed loops, the special formation of the loops
described as supra-csecal kinks and supra-duodenal loops, the great lengthening of the
colic cseca or their reduction to vestiges or to short compact glands, and the shortening
and straightening of the rectum *, are apocentric characters. That two birds possess

* Since writing this memoir, I have been interested by noticing that Professor Metschnikoff attributes many of the
digestive troubles of man to inheritance of a long rectum. The higher birds of every group apparently have passed
through a similar condition, but have succeeded in very greatly reducing the length of the rectum. — P. C. M.

40*

272 DE. P. CHALMERS MITCHELL ON THE

apocentricity in an organ is no evidence of their affinity. In the first place, the apocen-
tricity may be the mere result of growing the same inherited " germs " in similar
culture media. I call such apocentricities multiradial, implying that they are the result
of similar conditions on the same plastic material. The lengthening of the whole gut
and the spiral twisting of portions of it, and in particular the lengthening of the cseca,
are plastic or multiradial effects and can have no direct bearing on affinity. The
extreme shortening and widening of the whole gut and the disappearance of the cseca,
or even their transformation to nipple-like excretory organs, are multiradial. The
production of a supra-duodenal loop and its retention after shortening of the long caeca
in connection with which it arose are multiradial apocentricities. These again give us
no clue to affinity. There is no reason to suppose that even in the actual phylogenetio
tree of birds, a branch the members of which now possess an organ with archecentric
character, may not have come from a branch the members of whicli now possess an
apocentric character in the same organ. For, in the first place, the apocentricity may
have arisen after the branching; and, in the second place, if the apocentricity be truly
plastic, the transmitted germs in another environment may grow only into the ancestral
form. It is probable, however, that apocentricities, even if multiradial, leave some
legacy of complexity in their simplified descendants, and such conditions of character I
have called pseudoceutric. The conclusion to which consideration of multiradial apocen-
tricity leads is that even if the phylogenetic trees based on the study of the intestinal
tract be absolutely correct, and if they be compared with equally correct trees based on
the examination and valuation of other characters, these trees may not coincide.

Apocentricities, however, may be so definite;, so precise, or anatomically so complicated,
that they appear to imply a phylogenetic contraction of the range of variability in
respect to the structure in question. Such a demarcation of modification along a single
and definite radius I have called uniradial apocentricity. When further divergent
modifications occur on a single radius, there is formed what I have termed a metacentre,
and what seems to imply that the range of variability has been limited or defined, with
the result that future modifications all retain an indication of their more limited or
defined starting-point. I take it that the archecentre of the alimentary canal of birds
is a metacentre of the alimentary canal of the Sauropsida, and that, again, is a metacentre
of the alimentary canal of the whole vertebrate stock. I have tried to show that the
uniradial apocentricities of the intestinal tract of birds relate to the number and position
of the minor loops into which Meckel's tract is produced.

The obvious use of the study of characters as regards classification is, then, the
valuation of characters as archecentric and apocentric, and the discovery among the
apocentricities of the uniradial modifications with their metacentres. When these have
been determined and valued, the characters have yielded all the material that they afford
for genealogical enquiry. When one set has been exhausted, recourse must be made to
another set. In other words, the work of the anatomist whose goal is the achievement
of natural classification, is the study of the definition and limitation of variation within
groups. There is, however, another and most important side to this valuation of

INTESTINAL TEACT OF BIRDS. 273

zoological characters. When the archecentricities and apoceutricities of a number of
organs or structures have been mapped out, the questions are opened as to how far
archecentric conditions of different organs are combined in the same animal ; how far
apocentric conditions are so combined ; how far archecentric conditions of one set of
structures are associated with apocentric conditions of other sets. In general terms, the
problem is how far different organs become modified or remain unmodified independently
or in association in the same individuals. If we compare the Dog-fish and the Frog, we
find that, in relation to the general characters of the Ichthyopsida, the Dog-fish is arche-
centric as regards its skeletal system, apocentric as regards its urinc-genilal system,
while the Frog is apocentric in its skeleton and archecentric in its urino-genital system.
Naturally, in a paper dealing with part of one system, this problem of association has
not been raised, but I desire to add my conviction, based on a general study of birds and
a particular study of small groups of birds, that the larger the number of forms in a
group and the narrower the anatomical range within which the group is confined, the
more close is the association of apocentric modification of different organs.

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O:-

UNIVERSITY

OF

INTESTINAL TEACT OF BIRDS. 275

EXPLANATION OF THE PLATES.
PLATE 21.

Evolution of the Intestinal Tract in the Pelargo-Colymbomorphine Brigade.
(For Colymbimorphine read Colymbomorphine.)

PLATE 22.

Evolution of the Intestinal Tract in the Alectoromorphine Legion.

PLATE 23.

Evolution of the Intestinal Tract in the Coraciomorphine Legion.
(For Coracimorphine read Coraciomorphine.)

In these Plates the affinities of the conformations of the Intestinal Tracts and not of the birds are
indicated. The outline of the coils of the tract in the unfolded condition is given ; the Hue of attachment
of the mesentery to the body lies to the reader's right. The diverticulum is represented as a dark spot
on the left. The duodenum lies towards the upper end of the Plates, the rectum towards the lower end.
with the caeca, when present, as a pair of dark lines.

I?
OFTH I

UNIVERSITY

Chalmers Mitchell.

TRANS. LINN. Soc. ZOOL SER. 2 VOL WL PL 21.

/: 'Mfp'}

Gypdh'ierax /'

.. .

Falconiform ^/

•••••Ansehne
•••. Meiacentre
\ Iff

I ,..A A

.$te?anopoSV '*
'•Metacen

•./ '••. ...«• ;-
'•. Phcsmcdpteru5^/

Tachybaptes

Colyirilius

''•-. U

Pelargo-Cofembi'morphme
Met£centre

PC M del

ArchecenLre
Palamedea Ratitae

INTESTINAL TRACT OF BIRDS
PELARGO-COLYMBIMORPHINE BRIGADE.

Chalmers Mitchell.

TRANS. LINN. Soc. ZOOL. SER Z. VOL VIII. PI. 22

"••• ' ••"••" /$"~

ichciffiphus M/ Otididee /

'-Mi, Q!f«

Euryiygidae

""" ""

•v Thinoconj
\ Parrid*;

v^ /

Palainedea

Arohecentre

RC.M. del.

INTESTINAL TRACT OF BIRDS'
ALECTOROMORPHINE LEGION.

•CRalrfifers Mitchell.

TRANS. LINN. Soc. ZOOL. SER. 2 VOL VIIl: Pi. 23.'

CbraciWiiculiform
MMbentre

CiiciiTidas

(

Loracimotphine
Metpcentre

( Palamedea)
Archecentre

P. CM del

INTESTINAL TRACT OF BIRDS
CORACIMORPH1NE LEGION.

'

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ADD O 1 \QO 1

ArK 2 1 13o4

WI&Y181964

Vrt<?-f 3

7 J

JUN 3 19G4

28MY'64GW

D/

JUN/s 1979

Su^t to Recall

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LD 21— 50?n 4 '63 General Library

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U.C. BERKELEY LIBRARIES

164732

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